MRIaggr-calcGroupsW_cpp: Find spatial groups
In bozenne/MRIaggr: Management, Display, and Processing of Medical Imaging Data

C++ function called by calcGroupsW to compute the spatial groups. For internal use.

1	calcGroupsW_cpp(W_i, W_p, subset, max_groups, verbose)

`W_i`	the 0-based row numbers for each non-zero element in the sparse neighbourhood matrix. integer vector.
`W_p`	the pointers to the initial (zero-based) index of elements in the colum of the sparse neighbourhood matrix. integer vector.
`subset`	the subset of observations to use. positive integer vector.
`max_groups`	the maximum number of groups. postive integer.
`verbose`	should the execution of the function be traced ? logical.