Update get_obslist file handling

[mmccrackan]

Just a minor change to make it more clear that get_obslist accepts either a file input or obsdb query. Came up when #1676 was being tested.

[msilvafe]

I'm a bit confused by this PR...what exactly is this solving? In SAT-ISO we already routinely pass paths to a txt file with a list of obsids in it and it's handled just fine through the block:

try:
    with open(query, "r") as fname:
        return [context.obsdb.get(line.split()[0]) for line in fname]
except FileNotFoundError:

[mmccrackan]

I'm a bit confused by this PR...what exactly is this solving? In SAT-ISO we already routinely pass paths to a txt file with a list of obsids in it and it's handled just fine through the block:

try:
    with open(query, "r") as fname:
        return [context.obsdb.get(line.split()[0]) for line in fname]
except FileNotFoundError:

Mostly it was just to make it more obvious in the code that this function accepts a text file as input (wasn't in the docstring). I figured I'd just improve the handling of file inputs at the same time instead of just having it do a try except. There were no bugs in the implementation.