Add colorbar and an option to plot the velocities to plot_traj()

[T0T0R]

A color bar can now be added when plotting the trajectories, which is helpful with colorby='frame'. The title of this colorbar can be selected between "Frame number" or "Time (s)".
Example with tp.plot_traj(t, mpp=1.0, fps=24, colorby='frame', cmap='cividis', colorbar_units='frames', superimpose=background):

Example with tp.plot_traj(t, mpp=1.0, fps=24, colorby='frame', cmap='cividis', colorbar_units='real', superimpose=background):

Another feature is added, colorby='velocity' allowing to color the trajectories by the speed of the objects. The title of the color bar can be selected between "Speed (px/s)" or "Speed (µm/s)".
Example with tp.plot_traj(t, mpp=1.0, fps=24, colorby='velocity', cmap='inferno', colorbar_units='frames', superimpose=background);:

Example with tp.plot_traj(t, mpp=1.0, fps=24, colorby='velocity', cmap='inferno', colorbar_units='real', superimpose=background);:

[nkeim]

Thanks for this pull request! In the interest of keeping trackpy's API simple and memorable, I wonder if the colorbar_units argument is really necessary. It should be possible to choose the units automatically based on whether mpp or fps (or both) are given, which is currently the behavior for the position axes. This also avoids the ambiguity of the half-real units µm/frame and px/s.

Also, the current master branch fixes the automated checks which would be helpful for testing. If you're comfortable with rebasing this PR, please do so; no worries if not.