WIP adding gettotals-functionality to Nipype

Ged Ridgway's get_totals.m seems to be quite utilized piece of code for calculating structural volumes (global or ROI), but it seems to require full Matlab to run on. This is just a proof-of-concept test of implementing it into Nipype so that it can also be run in pipelines and in an environment with just SPM Standalone with Matlab Runtime. The current code it still lacks the threshold -parameter functionality, and I'm not sure if this approach taken is a proper one, or should it be implemented into some other module or not at all. So all comments would be very welcome!

Ref: original get_totals.m
http://www0.cs.ucl.ac.uk/staff/g.ridgway/vbm/get_totals.m