* Also rename corresponding C sources.

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

- removed some unnecessary variables

It was forgotten to pass the "global" variable to the arr_norm_2() function

- 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

CG with different preconditioners tested

This was necessary after rebase

To tidy up solver.f90 a bit, global_sum, global_norm_2, calc_rel_res, calc_abs_res
and global_dotproduct were moved to linear_algebra.f90