Feature/loopy invert matrix

Moves the matrix inversion stuff from blockstructured/geometry.py to pdelab/tensor.py. This touches on the subject of #139 (closed).

You can get the name of the inverse of a matrix A with shape shape from

name = name_matrix_inverse(A, shape, visitor)

matrix_inverse() returns the inverse wrapped in a pymbolic variable, although I don't know how useful this is.

TODO:

• handle inames correctly
• what about tags? -> not really necessary
• what about dependencies -> not really necessary