Allow local IERS file redux

[jchiang87]

For running at Theta, we need to reimplement the fix described at #200

[cwwalter]

It looks like relative to when we dealt with this before there is now a new option:

iers.conf.auto_max_age = None
https://docs.astropy.org/en/stable/utils/iers.html

which stops the check but would require us to prestige it in the "Standard" area, so I think the previous

iers.conf.iers_auto_url = 'file:/srv/finals2000A.all'

is still better assuming that it still works.

[jchiang87]

For this morning's tests at Theta, I did try adding just that line, but it still tried to download the file, so I needed to set the iers_auto_url to point to a local copy.

[jchiang87]

Apparently astropy also wants to download a leap second file:

Downloading https://hpiers.obspm.fr/iers/bul/bulc/Leap_Second.dat
|===========================================| 1.3k/1.3k (100.00%)         0s

We can provide an alternative location by also setting iers.conf.iers_leap_second_auto_url to a local file URL, but it's easier just to disable downloads entirely. This can be done with

from astropy.utils import iers
iers.conf.auto_download = False
iers.conf.iers_degraded_accuracy = 'warn'

[jchiang87]

This effectively closed by #427