`lieb_ando`: use partially specified problem to get correct `vexity`

[ericphanson]

This is in fact the approach used in CVXQUAD (https://github.com/hfawzi/cvxquad/blob/master/lieb_ando.m), but we didn't have those yet at the time.

This way we don't have to create an atom! Pretty convenient.

Also: I noticed in many of the CVXQUAD-derived atoms, we expect some variables to be PSD, but don't check or enforce it. Could be something we might want to do.

closes #683

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.20%. Comparing base (2978c32) to head (9441aef). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #684 +/- ## ======================================= Coverage 98.19% 98.20% ======================================= Files 89 89 Lines 5162 5168 +6 ======================================= + Hits 5069 5075 +6 Misses 93 93 ```

:umbrella: View full report in Codecov by Sentry.
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[ericphanson]

unexpected/unrelated docs failure:

ERROR: LoadError: LoadError: ArgumentError: Package TimeZones does not have RecipesBase in its dependencies:
- If you have TimeZones checked out for development and have
  added RecipesBase as a dependency but haven't updated your primary
  environment's manifest file, try `Pkg.resolve()`.
- Otherwise you may need to report an issue with TimeZones

[odow]

I just restarted it. The TimeZones release is only 2 hours old: https://github.com/JuliaTime/TimeZones.jl/releases/tag/v1.16.0, so I wonder if it's an issue with the cache

[odow]

Ha. I just arrived at https://github.com/JuliaTime/TimeZones.jl/pull/459#issuecomment-2123418212 to find you there too.

As a work-around, we can update the docs to build on Julia 1.10

[ericphanson]

yeah, that works. I put up a patch also: https://github.com/JuliaTime/TimeZones.jl/pull/461