The matrices are not in use by any process.

shape matrix cache is now stored in the ProcessData of the concrete process

Small optimization for first variable, no need to make a set_union
with empty _ids_of_active_elements.

 of the entries of the global residual vector that do not need
 initial non-equilibrium compensation

Use const references where necessary.

Rename a the method from getNumberOfComponents() to getNumberOfGlobalComponents()
in class LocalToGlobalIndexMap.