make it easy to interface with outside solvers

[rmodrak]

Gian Matharu has interfaced SeisFlows with a 2D finite difference solver. Starting to look through Gian's code for ideas on how to make it easy to work with external solvers.

[rmodrak]

So far, made some simple changes to the inversion workflow. I did, however, leave in the initialize method, which is useful, for example, if you want to implement source encoding or source decimation. See also thrifty_inversion workflow, which uses on initialize and, at the expense of some complexity, provides quite significant computational savings.

[rmodrak]

Next, will explore further if it is possible to decouple the data reader from the solver in the way Gian was suggesting.