Improve unit testing about seisflows

[bch0w]

Unit tests are still underdeveloped for this package. The superscripting approach, interconnected modularity, and reliance on external solvers and systems, makes this a difficult codebase to unit test appropriately.

I am chipping away at this but it may require some clever approach or a re-thinking of the internal implementation to detach the source code a bit more from external programs.

[bch0w]

Unit testing is now easier to implement thanks to #125, which also introduced a suite of unit tests that take advantage of PyTest. The test suite is still underdeveloped and could use some additional work.

[bch0w]

Leaving this comment as reminder to implement codecoverage and identify sections of the code that would benefit from improved unit testing, related to #120