Merge branch 'add-gpl' into 'master'

Added LGPLv3 to top-level of repo.

See merge request !730

COPYING.LESSER

+### 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.
+
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+than an Application or a Combined Work as defined below.
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+and utility programs needed for reproducing the Combined Work from the
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