Start documentation of MVT_flinn

Make code work with lists of vel grad tensors and
file names. Add basic documentation.
Add options to MVT_flinn

* 'flinn_only' to only plot the Flinn diagram
  and not the other subplots
* Colourtime' to colour time steps.
Allow user to set limits in MVT_flinn

Options 'max_e' and 'max_v' allow the
caller to set the maximum value of the
vorticity number and strain rate to be
plotted.