Update for new Visibilities object and some tidy

[samaloney]

• Upate internal code for new vis object
• Update imaging demo to use new vis object feature (slice, meta)
• Small tidy ups

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 47.45763% with 31 lines in your changes missing coverage. Please review.

Project coverage is 72.30%. Comparing base (2854ed9) to head (a847cc3).

Files	Patch %	Lines
stixpy/calibration/visibility.py	43.58%	22 Missing :warning:
stixpy/imaging/em.py	55.00%	9 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #124 +/- ## ========================================== - Coverage 72.57% 72.30% -0.28% ========================================== Files 31 31 Lines 1856 1892 +36 ========================================== + Hits 1347 1368 +21 - Misses 509 524 +15 ```

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