implement DiLeptonMatrixMethod::getIndexRegion

Details:
this takes into account the fact that the enum values do not
necessarily match the indices used to internally store the histograms
holding p(T|F) and p(T|R).

Before it was done creating these histograms for everything, plot
regions and control regions, although we used the fake estimate only
for plot regions.

Now at least that distinction is clear, but this implementation is
still insane. Note to self: here is how I think we should implement
it: forget the enums. Just decide elsewhere (i.e. in the plotter,
where you actually create and fill the histograms) what are your
signal regions and how you want to call them. Here we don't care, and
you should just do three things:
0) at initialization, pass in a list of strings for the regions where
   you want the fakes; check that we have all the histograms.
   When running, provide two access methods:
1) getFakeWeight( string regionName )
   This method will be slower, but we don't want to do premature
   optimization.
2) getFakeWeight( int regionIndex )
   This method will be faster, and let the user cache the index with
   something like getIndexRegion.