[la] proper specialisation mechanism for solver interface

allows to pass all matrix/vector types derived from the
Eigen{Matrix,Vector}Interface to pass into the eigen solvers
[fem]fixup matrix object
[la] allow setting ilut params via config
[la] removes Solver namespace
[la.solver] createSampleDescription -> defaultSettings
[la] removes container namespace
[la.solver] simplify apply interface
[la.solver] default bicgstab settings
[la] note broken direct eigen solvers
[la.solver] fixes default bicg settings
[la.solver] removes broken direct ones, adds eigen-superlu
[fem] fixup define guard in matrixobject
[la.eigen] better EigenRowMajorSparseMatrix reserve from pattern