Small changes in tensor on free modules + TestSuite issues fixed.

Class FiniteRankFreeModule:
renaming:
- basis_change() --> change_of_basis()
- set_basis_change() --> set_change_of_basis()

Class FreeModuleTensor:
- introduced components() with an alias to comp()

Class Components:
- new method _matrix_()

Class FreeModuleCoBasis now inherits from UniqueRepresentation

Fixed TestSuite issues on elements