Add other_fields to loader

[ykyohei]

Motivation is to load preprocess archive per observation easily.
The problem was preprocess archive is usually computed per (band, wafer) and each has scalar fields that cannot be concatenated.
One solution was to drop them by drop_fields , but number of fields to drop is large for ISO preprocess archive and it can increase.

This PR adds other_fields of aman.concatenate to loader to 'drop' them all at once or pick 'first'.

sotodlib/sotodlib/core/axisman.py

Lines 508 to 516 in 88a3230

	- If other_fields='exact' will compare entries in all items
	and if they're identical will add it. Otherwise will fail with
	a ValueError.
	- If other_fields='fail', the function will fail with a ValueError.
	- If other_fields='first', the values from the first element
	of items will be copied into the output.
	- If other_fields='drop', the fields will simply be ignored
	(and the output will only contain fields that share the
	target axis).

[mhasself]

Looks good to me!