Port the cal-table gains/D-terms split to the mixpol branch

[rohandahale]

Ports the cal-table gains/D-terms split (#331, merged to dev-backend as 357a8b9) onto dev-backend-mixpol, so the mixed-pol branch stops carrying the welded D-term row layout that #331 removed.

Cherry-pick of the #331 merge commit, plus one cleanup. No new cal-table work.

Resolutions

Two files conflicted, both because the mixpol copy differs from dev-backend for reasons unrelated to this PR:

• const_def.py: Caltable: split gains and D-terms into two tables #331 added DTDTERM to __all__. This branch has no __all__ (it is a dev-backend-only addition), so the block is dropped rather than introduced. Verified this costs no exports: with no __all__ the star import in ehtim/__init__.py re-exports everything, and eh.DTDTERM, eh.DTDTERM_CIRC, eh.DTDTERM_LIN and eh.upgrade_caltable all resolve.
• plotting/interactive.py: comment-only. The mixpol copy of the D-term panel TODO lacked the "in MixPol Phase 1" phrase, so only the removed side differed. Both branches now carry the same comment.

#331 removed the five Array/Obsdata legacy-upgrade tests from test_mixedpol.py, which left _legacy_circ_datatable() with no callers (it is dead on dev-backend today for the same reason). This branch drops the helper. _LEGACY_DTPOL_CIRC still has a live user and stays.

[codecov]

Codecov Report

❌ Patch coverage is 85.57214% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.17%. Comparing base (50784a5) to head (a8454b9).

Files with missing lines	Patch %	Lines
ehtim/caltable.py	85.35%	17 Missing and 6 partials ⚠️
ehtim/const_def.py	94.73%	1 Missing and 1 partial ⚠️
ehtim/calibrating/network_cal.py	0.00%	1 Missing ⚠️
ehtim/calibrating/polgains_cal.py	0.00%	1 Missing ⚠️
ehtim/calibrating/self_cal.py	0.00%	1 Missing ⚠️
ehtim/survey.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           dev-backend-mixpol     #333      +/-   ##
======================================================
+ Coverage               48.79%   49.17%   +0.38%     
======================================================
  Files                      54       54              
  Lines                   27132    27241     +109     
  Branches                 4631     4662      +31     
======================================================
+ Hits                    13238    13397     +159     
+ Misses                  12400    12324      -76     
- Partials                 1494     1520      +26     

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[achael]

looks good, sorry for the delay in approving