Add machinery to truncate keys

As part of data reduction to preserve a subset of p2654 after data archival, we want to remove parts of digitizer traces as these were significantly lower than necessary. The existing machinery with subslice-keys is not quite enough, as this assumes the same slice for both input and output.

This MR extends the subslice-keys operation to optionally have different input and output slices and adds a reshape-keys similar operation similar to the existing rechunk-keys to actually change the output's dataset shape before a partial copy. This is exposed via a new --truncate-keys option in the ManualRemoval plugin.