Automatic detector-panel removal

[GilesStrong]

 ## Problem

The optimisation will sometimes try to minimiser some panels, in order to assign their budget to other, more useful panels, however, this can lead problems, e.g. ScatterBatch._filter_scatters will remove any muons with at least one high-uncertainty hit, i.e. all muons get removed. Example:

Potential solution

A callback that automatically removes panels. This could trigger on e.g.

• once their scaled x or y span drops below a fixed threshold
• if the number of filtered muons exceeds a threshold (absolute or relative to batch size)

[GilesStrong]

Potentially this could also be caused by an anomalous gradient making one of the panels jump in assigned budget; may need a step-limiting callback.