Add plotting functionality for velocity streamlines

[fmccormack]

In the matlab plotting interface, there is the option to plot_streamlines. It's built on the flowlines.m function, for which there is a python equivalent (~/src/m/exp/flowlines.py).

Would it be possible to build both the flowlines functionality into pyissm (perhaps in pyissm.tools.exp.flowlines), and then add the streamline plotting functionality ?

flowlines.py