Version 2.1.0: Secure Content Extension

Version 2.1.0 adds new functionality. The changes compared to version 2.0.0 are:

• Support for the Secure Content Extension of the 3MF format https://github.com/3MFConsortium/spec_securecontent/blob/master/3MF%20Secure%20Content.md. A minimal example for reading and writing is in the binary SDK (under Examples/Cpp/Source/SecureCube.cpp), as well as here in the tests: Tests/CPP_Bindings/Source/EncryptionMethods.cpp.
  Note that lib3mf does not provide the cryptographic algorithms specified in the Secure Content extension. The tests of lib3mf make use of https://www.libressl.org/.
• Write-support for the Production Extension with non-root models https://github.com/3MFConsortium/spec_production/blob/master/3MF%20Production%20Extension.md.
• Support for version 1.1.0 of the beamlattice extension
  https://github.com/3MFConsortium/spec_beamlattice/releases/tag/1.1.0
• A small number of bugs have been fixed.