fix(hfbcat): Re-initialise class variables (#86)

Because it subclasses from FluxCatalog, by default will inherit
the class variables from FluxCatalog, but that's where FluxCatalog
stores the catalogs it loads at import time.  (Which seems like a
weird, an un-Pythonic, effect of importing a module.)

The side effect is that all the FluxCatalog catalogs end up in the
HFBCatalog as well, which I suspect is not intended.  So here's
a fix to re-initialise these class variables to nothing, so that the
FluxCatalog catalogs don't pollute the HFBCatalog.

Now, only the catalogue(s) loaded explicitly into HFBCatalog will
exist in it.

ch_util/fluxcat.py

@@ -243,7 +243,7 @@ def _deriv_fit_func(x, *param):
 
 class MetaFluxCatalog(type):
     """Metaclass for FluxCatalog.  Defines magic methods
-    for the class that can act on and provice access to the
+    for the class that can act on and provide access to the
     catalog of all astronomical sources.
     """
 
@@ -444,7 +444,7 @@ def __init__(
                 if alt_name in self._alternate_name_lookup:
                     alt_source = self._alternate_name_lookup[alt_name]
                     warnings.warn(
-                        f"The alternate name {alt_name} is already "
+                        f"The alternate name {alt_name} for {self.name} is already "
                         f"held by the source {alt_source}."
                     )
                 else:

ch_util/hfbcat.py

@@ -39,6 +39,10 @@ class HFBCatalog(FluxCatalog):
         "freq_abs",
     ]
 
+    _entries = {}
+    _collections = {}
+    _alternate_name_lookup = {}
+
     def __init__(
         self,
         name,