Add capability to visualize SPECFEM2D models

[rmodrak]

There are now some instructions for SPECFEM2D visualization included at
http://seisflows.readthedocs.org/en/latest/instructions_remote.html#visualize-inversion-results

The '.bin' files themselves are Fortran binary files, which have 4 bytes padding at the beginning and end and data in the middle. Some relevant information is available here
http://stackoverflow.com/questions/8751185/fortran-unformatted-file-format

In the future, I hope to move to a more standard binary file format for models and kernels. The only reason we haven't done so yet is that it would require fairly significant modifications to SPECFEM2D.

[bch0w]

It would be good to have very simple (matplotlib) capabilities to visualize models, kernels and gradients from SPECFEM2D workflows, especially for the example problem. These figures should also be included in the walk through of the documentation page (https://seisflows.readthedocs.io/en/latest/specfem2d_example.html)

[bch0w]

This was solved with #134 and improved in #137