To/matrices

See merge request !58

Otherwise the compiler would complain for unsigned types, see also Array
and Vector methods.

- DistributedMatrixTest is still broken because the rowVectorProduct
  method is not implemented, see issue #65

- BiEllpack and ChunkedEllpack are broken because the "segmentsCount"
  attribute is missing, see issue #67

Adding __cuda_callable__ to getElement in SparseMatrix and SparseMatrixView fixes the diagonal preconditioner

Clang is KOKOT and did not like it...