Optimize usage of astropy.units

[abhisrkckl]

#1641

This is a superset of #1643.
So this should be merged only after merging #1643.
I am doing it this way to make sure that the performance comparisons are meaningful.

[codecov]

Codecov Report

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (11ccc5d) 68.55% compared to head (9930fc2) 68.60%.
Report is 15 commits behind head on master.

❗ Current head 9930fc2 differs from pull request most recent head 72354c3. Consider uploading reports for the commit 72354c3 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1644      +/-   ##
==========================================
+ Coverage   68.55%   68.60%   +0.04%     
==========================================
  Files         103      103              
  Lines       23528    23564      +36     
  Branches     4102     4113      +11     
==========================================
+ Hits        16130    16165      +35     
+ Misses       6381     6378       -3     
- Partials     1017     1021       +4     

Files	Coverage Δ
src/pint/fitter.py	83.83% <100.00%> (ø)
src/pint/models/binary_ell1.py	86.00% <100.00%> (ø)
src/pint/models/dispersion_model.py	88.42% <100.00%> (ø)
src/pint/models/fdjump.py	89.85% <ø> (ø)
src/pint/models/frequency_dependent.py	83.09% <100.00%> (ø)
src/pint/models/ifunc.py	85.71% <100.00%> (ø)
src/pint/models/noise_model.py	94.03% <100.00%> (ø)
src/pint/models/pulsar_binary.py	77.51% <100.00%> (ø)
...c/pint/models/stand_alone_psr_binaries/DD_model.py	88.19% <100.00%> (ø)
...int/models/stand_alone_psr_binaries/ELL1H_model.py	78.77% <ø> (ø)
... and 18 more

... and 1 file with indirect coverage changes

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[dlakaplan]

If you want to do this, then I think https://nanograv-pint.readthedocs.io/en/latest/coding-style.html should be updated accordingly so we have a record of what to do.
(note that the coding style currently also allows python 2.7: should be fixed).

[abhisrkckl]

Closing this because this is a superset of #1643, and that was found to be inadequate.