estimate ascending node crossing from .EOF files

[scottstanie]

Calculates the ANX crossing from a zip file and an EOF file.

Right now

• it doesn't make the calculation from the POEORB the default ANX time. this is noted in the unit test, which will have to change
• im doing a dumb interpolation and we should probably just use isce3.Orbit instead of linear interp. of nearest two times
• we should, at the minimum, add the failing test case where it’s off by 3 seconds

for anyone taking it over, feel free to push to my branch (or take it and start your own)

[scottstanie]

Implemented more fully in #121