MD: standardize `POS` field

[dwhieb]

Standardize the POS field, and extract relevant information to other fields.

• Store the lexical category in Sense.category.
• Store information about animacy in Lexeme.features.
• Store information about plurality in Lexeme.features or possibly in Sense.usages.
• Store information about verb type (imperative) in Lexeme.features or possibly Sense.usages.
• If the entry is a prefix, store the class it attaches to in Sense.category, the class it derives in Sense.derives (if it's derivational), and the type of morpheme in Lexeme.morphemeType.

[dwhieb]

This won't be done. Instead we'll focus on mapping MD entries to CW entries, OR on improving the MD entries to have one of Arok's standard POS codes.