add specific exception when ellipse construction attempted without opencv-python installed

[zacharyburnett]

Addresses spacetelescope/jwst#7409
related to spacetelescope/jwst#7414

This PR addresses the issue brought up in spacetelescope/jwst#7409; a lack of specificity when using jump.use_ellipses = True without an opencv installation. Instead of a UserWarning being printed at import time, a ModuleNotFoundError is raised in runtime if ellipse functionality is selected and opencv-python is not detected.

ELLIPSE_PACKAGE_WARNING = '`opencv-python` must be installed (`pip install stcal[opencv]`) in order to use ellipses'

Checklist

• added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)

[codecov]

Codecov Report

Base: 78.19% // Head: 78.45% // Increases project coverage by +0.25% 🎉

Coverage data is based on head (1dbb1af) compared to base (9d95972).
Patch coverage: 40.62% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
+ Coverage   78.19%   78.45%   +0.25%     
==========================================
  Files          24       24              
  Lines        4843     4855      +12     
==========================================
+ Hits         3787     3809      +22     
+ Misses       1056     1046      -10     

Flag	Coverage Δ
unit	78.45% <40.62%> (+0.25%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/stcal/jump/jump.py	14.05% <36.66%> (+1.91%)	⬆️
tests/test_jump.py	62.96% <100.00%> (+31.48%)	⬆️

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[zacharyburnett]

this message accompanies the ModuleNotFoundError only if opencv functionality is used without it being installed