They are not suitable for more than 2 devices/execution types; their design
breaks the Open-Closed Principle. Instead, a type template "Self" was
created, which allows to change any template parameter.

Fixes #46

The x and b vectors might have different type.

RowLengths was renamed to CompressedRowsLengths.
OpenMP support wads added to matrix-vector multiplication and to scalar
products.
CSR format can use Cusparse for SpMV.

Min. interations feature was fixed in the GMRES method.
Timers for explicit and semi-implicit timestepping were added.

Implementing the matrix formats tests.