Feature/mp vcpe element definition

Added MP-vCPE template constants and model builder.
Some changes in already existing classes have been required in order to re-use code.

Some re-ordering of SP-vCPE template classes have been done, too:
- Constants have been moved to sp package
- Classes have been renamed to include SP in the class name.

NOTE: This pull request conflicts with next one (#514). Please, merge one first and notify me, so I can fix the other one.