Unify datasets part 1

[samtygier-stfc]

Issue

Work on #2199

Description

This is the first step in unifying MixedDataset and StrictDataset. This first step moves all the implementation of MixedDataset into BaseDataset.

This required some changes that result in a small changes to the rest of the codebase
Forcing constructor arguments to named, because the current classes use different argument ordering
Forcing access to the recons through a property.

Testing

I have moved the MixedDataset and StrictDataset tests into core.
I have added a DatasetTest, currently for BaseDataset. This will duplicate tests from the other test classes, and eventually replace them.
I have left the MixedDataset and StrictDataset alone (apart from small API changes), to ensure that the classes behaviour is not changed.

Acceptance Criteria

Test loading a dataset and an image stack

Documentation

Not needed yet

[coveralls]

coverage: 73.09% (-0.03%) from 73.115%
when pulling cf7bf35 on 2199-unify-datasets-1
into 8cf298f on main.

[JackEAllen]

Having now tested a few times and gone over the code a few times, I'm happy to approve this. If you feel that a second courtesy review is needed, I would also agree with this given the code we are touching, otherwise can just merge in :)

[samtygier-stfc]

I think its all going to get a bunch more testing over the next few PRs, so I'd suggest merging now