Fix Basis implementation and GSpline derivative

[rafaelrojasmiliani]

The implementation of the basis must be fixed in order to reduce the code in their derived classes. We require

• Fast implementation of n-th derivative matrix (returned as a referece)
• Fast implementation of the n-th matrix of the sobolev norm

This may be related to #2 as efficiently would also require to define derivative matrices as static members, i.e. common to all instances of a given basis.

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