N-variable mutual info with truth implemented and tested

Did some minimal testing against your code, seems to work fine. Uncovered a few things:
1) just testing tempProb!=0 isn't always enough: log(tempProb) can still return -inf
2) even with reasonable binning and low (2-3) dimensions, lots of bins are zero. Using N-variable code might be easier in future because it only allocates bins as needed
3) eliminated a couple of unnecessary squares/sqrts in computing errors. Get same answer regardless
4) a ton of ROOT functions do not work as advertised. 
Will make simple mutual info without truth script Monday and make plots. Ignore my edits to MutualInfoSingleVar.C. git merge was doing something weird.