Support for new Baker et al. color scheme, and plotting flow on a particular axis

[aviziskind]

Updates for plotting voxelmorph flow fields with the neurite.plot.flow(...) function. Some notes on the changes:

• Use the argument cmaps='Baker' to use the newer Baker et al colorwheel.
• Use the argument mode='transformer' or mode='t' to automatically set a few parameters optimized for plotting flow-fields from transformers.
• Use the argument yaxis_invert=True to invert the y-axis instead of flipping the data (img_indexing=True).
• Use the argument reduce=n to downsample a particularly large flow field by a factor of 'n' (should be an integer)
• The neurite.plot.flow(...) creates a grid of flow fields. But sometimes you may want to plot a single flow field within a larger set of other (non-flow-field) plots. So I put the code that generates a single flow field on a particular axis in the function neurite.plot.flow_ax(ax=...). The neurite.plot.flow then calls this sub-routine when creating a grid of flow fields, preserving backwards compatibility for those who still want to create a grid of flow fields with flow(...), but still allowing you to plot a single flow field on a specific axis (using flow_ax(...)), and without duplicating code.