Unify datasets part 5

[samtygier-stfc]

Issue

Work on #2199

Description

This begins the work of simplifying use of datasets, now that they have not functional difference.

Rename BaseDataset to Dataset.
Remove menu differences.
Check if the dataset has a sample, rather than if it is a StrictDataset. (this fixes errors that could occur if you remove delete a sample from dataset)
Remove some type unions

Testing & Acceptance Criteria

MI starts and can open data.
"Load 180" should be enabled in the menu if a dataset is loaded.

Documentation

Not yet

[samtygier-stfc]

Also using this PR to explore the changes from #2325 . Removing a test has triggered the expected "No current image" message on Applitools.

[coveralls]

coverage: 74.342% (+0.005%) from 74.337%
when pulling 4cb12ea on 2199-unify-datasets-5
into 0948a45 on main.

[MikeSullivan7]

All tests pass and all code changes look reasonable. Opened and loaded in various datasets to make sure they load in correctly. The "load 180" dialog works correctly when a dataset is loaded in. Other parts of MI work as usual.