Handle no clock files better

[aarchiba]

Current master emits loud and unhelpful error messages when working with telescopes for which no clock file is needed. This PR fixes that and checks that these telescopes behave better.

[codecov]

Codecov Report

Merging #1309 (aaf3771) into master (a74b41d) will increase coverage by 0.03%.
The diff coverage is 46.15%.

@@            Coverage Diff             @@
##           master    #1309      +/-   ##
==========================================
+ Coverage   61.61%   61.64%   +0.03%     
==========================================
  Files          89       89              
  Lines       20165    20173       +8     
  Branches     3611     3614       +3     
==========================================
+ Hits        12424    12436      +12     
+ Misses       6966     6960       -6     
- Partials      775      777       +2     

Impacted Files	Coverage Δ
src/pint/observatory/__init__.py	56.66% <0.00%> (+3.75%)	⬆️
src/pint/observatory/observatories.py	100.00% <ø> (ø)
src/pint/observatory/topo_obs.py	76.75% <40.00%> (-2.92%)	⬇️
src/pint/observatory/clock_file.py	80.38% <57.14%> (+0.54%)	⬆️

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[aarchiba]

Improved messages: https://nanograv-pint--1309.org.readthedocs.build/en/1309/examples/check_clock_corrections.html

The ERFA messages are because Parkes has a clock correction at MJD 0.something. We zap corrections that are exactly at MJD 0 so they aren't fed through the Time mechanism and don't trigger ERFA warnings, but almost-zero we don't zap.

[dlakaplan]

This looks good to me. I'm happy to merge, or to wait until we discuss (not that there is much to discuss here).

[aarchiba]

I think it's uncontroversial, so if you're happy please go ahead and merge.