Implemeting heat equation benchmark.

Refactoring the Dirichlet and the Neumann boundary conditions.

Adding simple heat equation benchmark for CUDA.

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.

Parallel reduction in CUDA fixed.
Boundary conditions in linear system assembler fixed.
Min. iterations for linear solvers fixed.

Adding --fast-build optin for the build script to speed-up compilation.