added LICENCE file

+### GNU LESSER GENERAL PUBLIC LICENSE
+
+Version 3, 29 June 2007
+
+Copyright (C) 2007 Free Software Foundation, Inc.
+<https://fsf.org/>
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+This version of the GNU Lesser General Public License incorporates the
+terms and conditions of version 3 of the GNU General Public License,
+supplemented by the additional permissions listed below.
+
+#### 0. Additional Definitions.
+
+As used herein, "this License" refers to version 3 of the GNU Lesser
+General Public License, and the "GNU GPL" refers to version 3 of the
+GNU General Public License.
+
+"The Library" refers to a covered work governed by this License, other
+than an Application or a Combined Work as defined below.
+
+An "Application" is any work that makes use of an interface provided
+by the Library, but which is not otherwise based on the Library.
+Defining a subclass of a class defined by the Library is deemed a mode
+of using an interface provided by the Library.
+
+A "Combined Work" is a work produced by combining or linking an
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+
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+for portions of the Combined Work that, considered in isolation, are
+based on the Application, and not on the Linked Version.
+
+The "Corresponding Application Code" for a Combined Work means the
+object code and/or source code for the Application, including any data
+and utility programs needed for reproducing the Combined Work from the
+Application, but excluding the System Libraries of the Combined Work.
+
+#### 1. Exception to Section 3 of the GNU GPL.
+
+You may convey a covered work under sections 3 and 4 of this License
+without being bound by section 3 of the GNU GPL.
+
+#### 2. Conveying Modified Versions.
+
+If you modify a copy of the Library, and, in your modifications, a
+facility refers to a function or data to be supplied by an Application
+that uses the facility (other than as an argument passed when the
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+version:
+
+-   a) under this License, provided that you make a good faith effort
+    to ensure that, in the event an Application does not supply the
+    function or data, the facility still operates, and performs
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+
+#### 3. Object Code Incorporating Material from Library Header Files.
+
+The object code form of an Application may incorporate material from a
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+
+-   a) Give prominent notice with each copy of the object code that
+    the Library is used in it and that the Library and its use are
+    covered by this License.
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+
+#### 4. Combined Works.
+
+You may convey a Combined Work under terms of your choice that, taken
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+the Library contained in the Combined Work and reverse engineering for
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+    Corresponding Source and Corresponding Application Code. If you
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+    the manner specified by section 6 of the GNU GPL for conveying
+    Corresponding Source.)
+
+#### 5. Combined Libraries.
+
+You may place library facilities that are a work based on the Library
+side by side in a single library together with other library
+facilities that are not Applications and are not covered by this
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+choice, if you do both of the following:
+
+-   a) Accompany the combined library with a copy of the same work
+    based on the Library, uncombined with any other library
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+    is a work based on the Library, and explaining where to find the
+    accompanying uncombined form of the same work.
+
+#### 6. Revised Versions of the GNU Lesser General Public License.
+
+The Free Software Foundation may publish revised and/or new versions
+of the GNU Lesser General Public License from time to time. Such new
+versions will be similar in spirit to the present version, but may
+differ in detail to address new problems or concerns.
+
+Each version is given a distinguishing version number. If the Library
+as you received it specifies that a certain numbered version of the
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+Free Software Foundation. If the Library as you received it does not
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+may choose any version of the GNU Lesser General Public License ever
+published by the Free Software Foundation.
+
+If the Library as you received it specifies that a proxy can decide
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+Library.