Enhancements

[j1c]

This is a list of things that can be changed to substantially improve performance of m2g. Some of it is addressed in #398

• Im 1000% percent sure below line is tabbed incorrectly. It should be outside the for loop.
  

  m2g/m2g/graph.py

  Line 216 in be29587

  self.g.add_weighted_edges_from(edge_list)

with ~1 million streamlines, it takes ~35 minutes vs 2 minutes with the line outside the for loop.

There is a parallel version in my PR.

• I think the streamline generation can be parallelized by sending partial seed list to bunch of Track instances.