Added options for mrtrix_track_roi2roi parameters, that allow setting FA threshold, curvature threshold and length threshold

We (Franco and Dora) tested back compatibility. The function can be called the old way without errors. The new parameters are passed in to the function as a varargin input, a series of if statements unpack the parameters. Default parameters are unchanged form previous version of the function.
I don't think you want this line to be here