Sensibly handle unfittable params, No unfittable params in plk

[abhisrkckl]

Unfittable parameters are parameters for which no derivatives are defined so that they cannot be fit.

1. Fitter objects should not be created any unfittable parameters are unfrozen. Currently, it fails with an unclear error message while calling fit_toas().
2. TimingModel.designmatrix() should fail with an informative error message upon encountering free unfittable parameters. Currently, it fails with an unclear error message.
3. pintk interface shouldn't list unfittable parameters. This will help unclutter pintk.

[dlakaplan]

The errors here might be due to function parameters which are visible but not intrinsically part of the model. So maybe just a check for parameter type as well.

[abhisrkckl]

The errors here might be due to function parameters which are visible but not intrinsically part of the model. So maybe just a check for parameter type as well.

A subset of the errors were indeed indirectly related to funcParameters. But they were actually due to a bug in the ELL1k model which I have fixed.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (9e21a05) 68.34% compared to head (fb14307) 68.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1648      +/-   ##
==========================================
- Coverage   68.34%   68.34%   -0.01%     
==========================================
  Files         104      104              
  Lines       24101    24112      +11     
  Branches     4277     4280       +3     
==========================================
+ Hits        16473    16480       +7     
- Misses       6545     6547       +2     
- Partials     1083     1085       +2     

Files	Coverage Δ
src/pint/models/binary_ell1.py	86.18% <100.00%> (+0.18%)	⬆️
src/pint/pintk/plk.py	13.51% <ø> (ø)
src/pint/fitter.py	83.63% <0.00%> (-0.21%)	⬇️
src/pint/models/timing_model.py	83.82% <50.00%> (-0.13%)	⬇️

... and 3 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[abhisrkckl]

Plk interface before excluding unfittable params:

After:

[dlakaplan]

This looks good. Is it ready?

[abhisrkckl]

Yes.

[dlakaplan]

Should I merge this?

[scottransom]

Sounds good to me!