Adding simple heat equation benchmark for CUDA.

The CXX_FLAGS variable does not exist/is unused by cmake.

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

Scripts tnl-compile and tnl-link 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.

Adding OpenMP support.