Allow a subset of dimensions to be passed to `selected_keys` argument in base calculators

[jwa7]

Currently, the user needs to know a priori the output keys returned by a given calculator's compute function to make a valid selection using the selected_keys argument. This is inconvenient for controlling, for instance, computation of blocks for a given subset of atom types.

It would be useful to be able to make key selections on a subset of dimensions. This is similar to the issue raised in #312 , but is requested for all calculators in general.