Array2root prototype

@noel I have some free time yesterday. I wrote a working prototype of array2root converter. I don't think I will be able to polish it in a short time (need to finish thesis). It's exactly the same mechanism as the other way around. The rest of the work would require some attention to details but are fairly trivial. If anyone interested in taking over and polish it, please do so. Here is what's left to do

- [ ] detect object column that's numpy branch and do appropriate action (I'm thinking of dumping it as vector)
- [ ] detect fixed length numpy array
- [ ] taking care of boolean
- [ ] and 16bit float from numpy
- [ ] write a proper wrapper to PyRoot (currently it returns TTree)
- [ ] write a proper wrapper to write directly to rootfile without touching PyRoot
- [ ] and document it