Consider moving freeze_iers to desiutil

[weaverba137]

freeze_iers() currently has two purposes:

1. The normal purpose of preventing code on a compute node from being forced to download the IERS-A table.
2. The extra, workaround purpose of preventing all code from downloading the IERS-A table from a server that is unavailable.

This creates a dependency on desisurvey for code that would not normally ever need that dependency. If freeze_iers() were moved to desiutil, this extra dependency would be unnecessary.

[dkirkby]

There is a third purpose, which was the original motivation and the reason it currently lives in desisurvey: ensure exactly reproducible coordinate transformations over the survey duration for the purpose of scheduling tiles in forecasts and operations.

[weaverba137]

Good point, but that would not prevent us from moving the code to desiutil.

[weaverba137]

In addition: from recent tests on desisim, it looks like freeze_iers() is not compatible with astropy 4.0. Make sure to test this if/when this moves to desiutil.