Refactor validators to reduce code duplication between bboxes and poses

[sfmig]

Is your feature request related to a problem? Please describe.
When implementing the ValidBBoxesDataset class we noticed there is some overlap with the ValidPosesDataset class.

Describe the solution you'd like
@niksirbi suggested defining a ValidMoveDataset base class which can validate what's shared across poses and bboxes. Then, ValidPosesDataset and ValidBBoxesDataset can inherit from the base, and only do the "extra" validations specific to each dataset type.

Describe alternatives you've considered
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Additional context
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[sfmig]

We could consider just having one type of dataset with relaxed validation (just requiring time and space)