Rewrite tests using Catch2

As a user of the library, I would like some proof that the library does what it says it does. I'd also like some advanced examples of how to use the library.

The current test suite suggests it does, but its spaghetti-layout makes that impossible to verify by eye.

Rewriting the tests for the core features needed to implement ReadSEGY and WriteSEGY with a framework like Catch2, with clarity of test cases in mind, should fulfill these needs.