Feature/linear systems

- Vector<const T> should now be supported
- CRSMatrix data is constant

It inferred wrong template deduction for index type
For instance
``
SparseMatrixDescriptor S(5);
``
implied `SparseMatrixDescriptor::IndexType` to be `int`

SparseMatrixDescriptor is also renamed from FlexibleMatrix. This is more clear
since it is actually not a matrix but a tool to allow simple construction of
matrices.

Also perform a few cosmetic changes (matrix classes have now template
parameters)

No preconditionner is used ATM

Add two kinds of matrices:
- FlexibleMatrix which are dedicate to filling/construct matrices
- CRSMatrix which are built from FlexibleMatrix and which should be fast (and
multi-thread compliant)