NiLabels - extra features removed to keep the library goal oriented

Final name: NiLabels.

Closer circle was drawn: external features not strictly related with labels measurements and manipulations had been removed

• ICV estimation moved in external repository: intra-cranial-volume-estimation
• Dummy generator moved in external repository (was only there for ICV testing): DummyForMRI

In progress (in that order)

• 80% coverage
• move to python 3 back compatible with python 2.7
• pip installable
• integrate in NiPy ecosystem.