Refactor sparse_mod

[krystophny]

Right now, only sparse_solve_method = 2 or 3 is allowed. sparse_solve_method=1 was previously used for SuperLU and the branch eliminated from the code in #15. Currently and previously there is/was no error handling.

[krystophny]

old docs:

!------------------------------------------------------------------------
  ! Winny: sparse
  !------------------------------------------------------------------------
  ! if sparse_solve should talk
  !  sparse_talk= .TRUE. ! default .FALSE. - neo2.in - settings
  !
  ! sparse method - only 1 (SuperLU) implemented
  !  sparse_solve_method = 1 ! default 0 - neo2.in - settings
  ! if sparse_solve is called with (sparse_solve_method .eq. 0)
  !  program stops
  !  one should call a normal solver in this case
  !
  ! These are three possibilities to call sparse_solve
  !  CALL sparse_solve(nrow,ncol,nz,irow,icol,val,b)  ! full column index
  !  CALL sparse_solve(nrow,ncol,nz,irow,pcol,val,b)  ! column pointer
  !  CALL sparse_solve(A,b)                           ! full matrix
  ! results are returned in b
  !
  ! Input
  !  INTEGER :: nrow,ncol,nz,nrhs
  !  INTEGER, DIMENSION(:), ALLOCATABLE :: irow,icol,pcol
  !  REAL(kind=dp), DIMENSION(:), ALLOCATABLE :: val
  !  REAL(kind=dp), DIMENSION(:,:), ALLOCATABLE :: A
  !
  ! In/output
  !  REAL(kind=dp), DIMENSION(:), ALLOCATABLE :: b
  ! or
  !  REAL(kind=dp), DIMENSION(:,:), ALLOCATABLE :: b
  !
  ! Matrix - square
  !  ncol = nrow
  !  ALLOCATE( irow(nz) )     ! row index
  !  ALLOCATE( icol(nz) )     ! column index (column ordering)
  !  ALLOCATE( pcol(ncol+1) ) ! column pointer (other possibility)
  !  ALLOCATE( val(nz) )      ! values
  !  ALLOCATE( A(nrow,ncol) ) ! full matrix will be converted to sparse
  !
  ! rhs
  !  ALLOCATE( b(nrow) )
  ! or
  !  ALLOCATE( b(nrow,nrhs) )
  !
  !------------------------------------------------------------------------

  !------------------------------------------------------------------------