Structure until now:
--------------------

grdchk_main:

- save unperturbed cost function
  fcref = fc

- choose an index
  -> done by equidistant loop
     do icomp = nbeg, nend, nstep
       ichknum = (icomp - nbeg)/nstep + 1

- grdchk_loc:
  determine entry within given field, i.e. 1-dim array, determined by
  * ctrlvarindex
  * ncvargrd   (ctrlvarindex)
  * ncvarnrmax (ctrlvarindex)
  * ncvarrecs  (ctrlvarindex)
  -> determine
  * icvrec 
  * jtile 
  * itile 
  * layer 

- grdchk_getxx:
  get control variable component from file
  -> only relevant for optimcycle > 0, i.e. when
     xx_... has values .NE. 0

- grdchk_setxx:
  * get control variable component (read from file)
  * add perturbation
  * write perturbed field back to file


- want to reset perturbation component after grad. check