Simplify adding new schemes

Adding a scheme requires modifications in multiple Python build files, which should not be the case.

Steps to add a scheme :

• Add src/schemes/XXX
• Add test/example/XXX
• Add src/params/XXX
• Add script/templating/XXX.yml
• Update preprocessing_src_cmake.py
• Update preprocessing_src.py
• Update preprocessing_src_schemes.py

Potentially other modifications such as modifying rbc_lib.py, test/test_kat.py for excluding the scheme from KATS, or test/test_memory_leak.py for excluding the scheme from memory leak testing.

Potential solution: Create configuration files (for build related options) for each scheme and make preprocessing generic using these new configuration files.