Provide a default WCS for ramp fitting.

[schlafly]

Units test started failing when we made the WCS a required argument, because the unit tests relied on being able to construct ImageModels from default RampModels. The latter don't have default WCSes, but the former require them, so the code to construct the one from the other needs to add a default WCS.

Checklist

• added entry in CHANGES.rst under the corresponding subsection
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.74%. Comparing base (1dc0501) to head (f91a3ff).
Report is 443 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1072   +/-   ##
=======================================
  Coverage   76.74%   76.74%           
=======================================
  Files         105      105           
  Lines        7013     7013           
=======================================
  Hits         5382     5382           
  Misses       1631     1631           

Flag	Coverage Δ		*Carryforward flag
nightly	63.01% <ø> (-0.02%)	⬇️	Carriedforward from 39a4d7e

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[nden]

This will do it.