Get obsid from uvh5 files

[plaplant]

This PR gets an obsid from a uvh5 file. Instead of being explicitly stored on the object, we use the GPS second corresponding to the first time in the observation. This is self-consistent with how hera_mc calculates the obsid.

[pkgw]

👍

Small stylistic comment, for functions with control paths like get_obsid_like_path, I like to structure them more like:

if caseA:
   return resultA

if caseB:
   return resultB

return None

… rather than nesting if-else's. But I doubt we're ever going to have more than two cases in this function, so, meh.