Adding diffuse map

[rlbyrne]

Adds the diffuse map from Byrne et al. 2021 into the catalog_data folder.

The map is formatted to be read in with the diffuse_model keyword. It is in a custom .sav file format. In the future, we should allow FHD to read more standard diffuse formats like pyradiosky's skyh5 format (see #254).

The map only contains Stokes I, Q, and U. Stokes V is set to zero everywhere.

Note that Stokes Q and U likely will not match the data well unless the ionospheric RM is accounted for (see #255).