Removed unusable constructor.

The constructor does not allocate memory. Furthermore it is not possible to allocate memory later on.
Added new class MatrixSparsityPattern.
Added constructor using MatrixSparsityPattern.
Made constructors of CRSMatrix explicit.
Replace nondependent lookup of _n_rows by dependent.
add static_assert to check if the code is compiled with _OPENMP
exclude MatVecMultNDPermOpenMP.cpp from build if OpenMP is not found
Merge pull request #132 from norihiro-w/fix-MatVecMultNDPermOpenMP

fix compiling errors when _OPENMP is undefined
Merge pull request #129 from endJunction/SparseMatrices

Sparse matrices