Implement forMatchingSignatures for efficiency

Implement forMatchingSignatures for efficiency

EC::Manager::forMatchingSignatures is different from
EC::Manager::forMatchingSignature in that it takes multiple signatures
and functions and iterates through all entities once. The trade-off is
that a vector of vectors is created to store matching entities.
This function can be called to use multiple threads as well, similar
to the "non-plural" version of this function.

See the doxygen-style documentation in src/EC/Manager.hpp for
forMatchingSignatures (or the generated doxygen html) for how to use.
Usage example is in the last unit test function in src/test/ECTest.hpp .