[operators] copy matrix in NumpyMatrixOperator.projected_to_subbasis
[operators] copy matrix in NumpyMatrixOperator.projected_to_subbasis

This increases performance of the error estimators, as matrix-vector
products are faster when the matrix is in a contiguous block of memory.

michaellaier/pymor,michaellaier/pymor,michaellaier/pymor,michaellaier/pymor