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 .
Unordered map does not preserve order in which items were inserted.
This is a problem because an entity id relocated to a previously deleted
one could be read as deleted when iterating through the unordered map.
Thus, it has been replaced with a queue.
clearSomeMatchingFunctions removed for
keepSomeMatchingFunctions and removeSomeMatchingFunctions to
avoid confusion.
Lines now respect 80 column limit.
An unordered_map stores functions, allowing for functions to be
removed by index, and bulk removals with filtering by index.
Also added reset(), which changes the state of the Manager to be
almost identical to a newly constructed one.
