Gradient test

[mloubout]

Rewrite of the Acoustic and TTI operators and codegen to separate operator, model and data.
Gradient test is also back in even so it may be too memory expensive for travis.

[mloubout]

Currently fails because of memory and permission error for deployement

[mloubout]

Only fails because of memory on linux travis, apple passes

[mloubout]

@navjotk @mlange05
This one need review.

Time_substitution is still there as this was created from Zhang.

I made a change to operator to output the structure (TimeData,...) instead of the data. It allow to pass back to gradient directly this structure and avoid useless memory copies. There may be a better way to do it than what I did but this is the main idea.

Finally i moved most of the u,v,rec,... to the operators in fwi_operators instead of Acoustic_codegen as most of it are temporary or input variable there is no reason to have as a property of the operator as it's only needed at computation if called.

[FabioLuporini]

We should really try hard to have lines shorter than 90 characters

[FabioLuporini]

I'll go through this properly later, but we should start having smaller, lighter PRs

[mloubout]

@FabioLuporini
This one is really hard to make smaller. In practice there is not much as it is only on test (gradient), but it required some rewrite to be able to run it properly without blowing up everything.

[mloubout]

The inversion repo is based on this branch so @vincepandolfo (because you worked already on inversion an looked a bit a this branch) and @mlange05 can we get this one going.

[vincepandolfo]

Use logger module here

[FabioLuporini]

One shouldn't use _ for class names -- I think it's quite a common rule in Python