Add ability to specify image domain measurements

[ptiede]

Eventually, it would be nice to fold in image-domain measurements that aren't actually constrained in VLBI, e.g., absolute flux, absolute positions, SED measurements, etc.

This adds experimental support for such features. Instead of producing only visibilities for each model, we also generate the total on-sky image. Some image domain models are so we only compute the image when necessary.

[codecov]

Codecov Report

❌ Patch coverage is 92.22222% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.46%. Comparing base (946c5b1) to head (4b6374e).
⚠️ Report is 68 commits behind head on main.

Files with missing lines	Patch %	Lines
src/observations/dataproducts.jl	66.66%	2 Missing ⚠️
src/skymodels/fixed.jl	50.00%	2 Missing ⚠️
src/skymodels/multi.jl	77.77%	2 Missing ⚠️
src/skymodels/abstract.jl	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #438      +/-   ##
==========================================
- Coverage   92.43%   88.46%   -3.97%     
==========================================
  Files          33       40       +7     
  Lines        1823     2515     +692     
==========================================
+ Hits         1685     2225     +540     
- Misses        138      290     +152     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ptiede]

Test failures are due to an Enzyme issue see EnzymeAD/Enzyme.jl#2873

[ptiede]

To fix the Enzyme issue I placed an upper bound. I'll relax that in the future when things work again.