Deprecate SeqPardiso and remove test_pardiso.

The preconditioner SeqPardiso using the sparse direct solver library Pardiso has not been actively maintained for several years. It is still based on a very old Pardiso version; finding the library never went into the CMake build system. Moreover, Pardiso is not open source. With SuperLU and UMFPack, two working alternative direct solvers are available.

Drop the support for Pardiso by deprecating SeqPardiso and removing the corresponding test.

Fixes #17 (closed). Please also port to 2.6.