Improve basionym detection

[mdoering]

we skip epithet groups that have multiple basionyms like this one:

• Motacilla maura Pallas, 1773
• Muscicapa maura Pallas, 1773

In this case both are synonyms for Saxicola maurus (Pallas, 1773).
The consolidation therefore will link all newly made synonyms to S. maurus anyways.
Suggest to then pick one of the names as the basionym and establish new links. Prefer the highest prio name, otherwise randomly pick one.

[mdoering]

Similarly we have original spellings or other orth variants listed as synonyms under the actual basionym:

• Larvivora sibilans Swinhoe, 1863
• Lavivora sibilans Swinhoe, 1863

[mdoering]

There are also cases within the same genus that we might want to ignore:

• Erythropygia vulpina Reichenow, 1891
• Erythropygia leucophrys vulpina Reichenow, 1891

It is rather unlikely that the same author, in the same year, published a species and a subspecies based on different types.
The Multiple basionym issue should only be used for epithets across different genera in basionyms.