TST: add test_eopc04_1962-now.py

[neutrinoceros]

Not done yet, but opening early as a warm up round.
This is the first piece towards #21

[pllim]

Why remove the init? If you don't want the init, might as well move the test folder to root dir?

[neutrinoceros]

Why remove the init?

it serves no purpose, besides maybe forcing git to track an otherwise empty dir.

If you don't want the init, might as well move the test folder to root dir?

sure thing

[neutrinoceros]

this is now ready for review.
Tests themselves should be straightforward to read, and I tried to keep the parsing logic that supports them intentionally brittle and 100%¹ covered. I made sure the only dependency, besides the stdlib, is pytest, and focused my testing on two crucial areas: the header, and the last 256 lines (which should generously cover all added lines in auto updates).
There's also some logic to check for value ranges by specifying optional (lower, higher) bounds for each columns. These are minimally exercised for the first column at the moment, and can easily be added later (or during reviews) if needed.

Footnotes

1. some branches are intentionally unreachable and marked as such. If they're reached, it'll signal broken test logic. ↩

[neutrinoceros]

actually checking all data elements didn't turn out to be prohibitively slow as I first expected, so the check is now much more extensive

[neutrinoceros]

@astrofrog for reference, I plan to wait until my approach here is validated (or not) before I add more tests. Ideally I would like to have tests for every data file receiving auto updates, but the cost of building them should get much lower if I can reuse at least some of what I'm proposing for this first PR.