No longer formatting microseconds into SSA time literals

[msdemlei]

This started as an attempt to fix bug #350; it turns out it doesn't do that and the heasarc SSA has other problems. But SSA 1.1, sect. 4.1.1.4, essentially implies we shouldn't be producing microseconds, so by the golden rule of interoperability just let's not.

[codecov]

Codecov Report

Merging #351 (4cc94ec) into main (ea07aed) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #351      +/-   ##
==========================================
- Coverage   78.28%   78.26%   -0.02%     
==========================================
  Files          46       46              
  Lines        5493     5494       +1     
==========================================
  Hits         4300     4300              
- Misses       1193     1194       +1     

Impacted Files	Coverage Δ
pyvo/dal/ssa.py	55.75% <0.00%> (-0.25%)	⬇️

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[tomdonaldson]

Thanks, @msdemlei. This seems like a reasonable work around and looks good to me. The tests for SSA are a bit sparse, but that was a preexisting condition and seems out of scope for this minor change.