Per-datatype rechunking control

[JelleAalbers]

This allows a multi-output plugin to specify which of its output dtypes should be rechunked. You can set the rechunk_on_save attribute to:

• True: rechunk all outputs
• False: do not rechunk anything
• (immutable)dict mapping dtypes -> True/False, per-datatype control.

The main use case is PulseProcessing in straxen, which outputs small monitoring datatypes (pulse_counts, veto_regions) besides the big records dtype. We can't rechunk records since that would make their expensive saving operation non-parallelizable, but we should rechunk pulse_counts and veto_regions to avoid saving many 3 kB files that give rucio a headache.