Remove N_u_op from ip data in THM and all other mechanical processes

The vectorial shape function operator N_u_op stored in the integration point data array uses (depending on the dimension 2 or 3) 1/3 or 1/2 of the integration point data size. Calculating it in the assembly "on-the-fly" reduces memory requirements significantly (analysis will be in discussion below).

Implementing a Eigen's CwiseNullaryOp creating a block-diagonal view on a matrix.

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