Refactoring peaks calculations to be out of odf.py

Hello,

We had problems with relative imports between the odf.py and the shm.py. For this reason I had some functions with imports inside the peaks_from_model and that was ugly and I feel the multiprocessing module didn't like too ;) 

Therefore, I created a separate file for the peaks calculations (peaks.py) and I updated the tests and the examples. Now, things look much cleaner and I hope that could solve the problem that we are having with the peaks_from_model failing in the Mac's. Let's see.

I shouldn't forget to say that in this PR I am also fixing a bug that we found together with Max. In peaks_from_model if return_sh is True I am also saving the matrix that can give back the ODFs from their SH coeffients. So, you basically get the ODFs for free if you have the SHs (which need much less memory). Unfortunately, before I was saving the inverse of the correct matrix. Now fixed :)

GGT!