* Also rename corresponding C sources.

* This is only one step mandated by the brain-dead behaviour of the
  NAG and PGI compilers respectively.

- IBM xlf doesn't allow for this kind of functions
- The matrix stencil now has a shift property to simulate the old behavior
- Additional shifted versions of all matrix stencil were necessary

- Corrections for IBM Fortran Compiler

- all solvers now use the functions from linear_algebra like arr_dotproduct arr_norm_2 etc.
- in linear_algebra.f90 renamed mat_* functions to arr_* functions (more meaningful)
- all spectral methods now take an additional extent as argument

- lambda_maxiter and _tol for functions related to eigenvalue calculation
- cheby_miniter and _checkrate for break condition in chebyshev iteration

CG with different preconditioners tested

This was necessary after rebase

Internal datastructures are now in solver_internal.f90 which shouldn't be imported
by external applications.