See #399

I tried to interpret and fix the code and for me it works nicely with 1 to 3 CVs.
Here a brief description: the index i spans through the whole reweghting-grid (according to the mpi rank in case of mpi).
rewf_grid_ contains the size on each dimension of the reweghting-grid. t_index is the array containing the coordinates of i in each dimension of the grid. The indexes had to be changed from i to j in line 1967, which indeed spans correctly the dimensions of the grid. In 1968 in order to get the correct index, we need to subtract the former index (ncv-2 -- instead of "ncv-1" which is yet not assigned -- from what is left of kk).

I didn't attempt any further test.

Please let me know if I had misinterpreted the code.

Fixes #399

cc: @valsson

cc: @carlocamilloni

Notice that I had to change to interface of a couple of functions
ActionSet:getLabelList() and getLabelVector()
adding a template parameter.
Should be harmless since these functions were used in a few places only.

and improved regtests