Add HFB target list as new catalog

[rikvl]

• Adds the JSON catalog of HFB test targets to ch_util/catalogs
• Adds this catalog to source_dictionary, so HFB test targets are available by their name (B/J numbers)
• Creates new class HFBCatalog, subclassing FluxCatalog, to make other properties stored in JSON catalog available

[rikvl]

There is a slight overlap between the existing collection specfind_v2_5Jy_vollmer2009.json and the new hfb_target_list.json added in this PR. This results in a bunch of somewhat annoying warning messages upon importing hfbcat.py (a warning for each of the alternate_names that already exists). It's these 2 sources:

• 3C_190 = B0758+143
• 4C_39.56 = J1821+3942

One solution would be to make things so one can import the class definitions from fluxcat.py without automatically loading the default collections. Is that desirable? We can also just live with these warning messages.