Support partial I/O

[piyanatk]

Supporting of partial I/O, at the very least over frequency, would be really useful for memory management when working with a large sky model with full spectral type.

This will be especially useful for a sky model that requires healpix component type, such as GSM or EoR maps, which is often not possible to be made as a spectral_index spectral type. Reading in this model made to contain the full HERA band would require a large amount of memory, for example. Our current workaround is to make one model per frequency, but this makes working over more than one frequency channel difficult. A partial I/O support would solve this.