Anisotropy

[mloubout]

@navjotk , @ggorman
I added three notebooks
VTI,VTIdens and TTI

It contains (for the moment) the coupled PDE to solve in the anisotropic case, I am looking into how to set it up in python and define the FD scheme. Ideally only TTI is useful but makes is easier to get an handle on coupled PDE with VTI.

[mloubout]

I also need to find out (I am checking with Philipp from my group) how the source injection and receiver measurement should be handled