new TFR code

starts to implement what is described in : https://github.com/mne-tools/mne-python/issues/1333

for now only sensor space analysis is investigated.

it also adds a new dataset (somato) well adapted to TF analysis.
The size is about the plot size while res is about the resulution of the meshgrid used for interpolation. It makes sense to have both params. I see a different problem, in many figures the other params are not coupled to the figures size. For example you make huge figure but the lines stay small, etc ...