sorting: sorted_index_list(): added array mask

* sorting: sorted_index_list(): added array mask
  * added dummy variable for array mask
  * take only into account array elements that are masked as true
  * use like mask= in e.g. minval()
sorting: sorted_index_list(): added dummy variable index_n

* sorting: sorted_index_list(): added dummy variable index_n:
  * intent(out), optional - returns number of unmasked elements
statistics: added masks to compute_median() and compute_stdev()

* statistics: added masks to compute_median() and compute_stdev():
  * use only array elements that are masked as true
Statistics: renamed functions compute_median(_sp)() and compute_stdev(_sp)()

* Statistics: renamed functions compute_median(_sp)() and compute_stdev(_sp)()
  * from: compute_median(), compute_median_sp(), compute_stdev(), compute_stdev_sp()
  * to median(), median_sp(), stdev(), stdev_sp()
  * kept wrappers for the old names