remove feature_id from ExpressionLevel

[saupchurch]

removes feature_id from ExpressionLevel and changes search to filter by name

[ejacox]

Thanks @saupchurch.
+1

[david4096]

Thanks @saupchurch +1. This change satisfies the same access patterns of resolving a feature and then searching the RNA expression database, except that one does not need to have a fully formed Feature record present to access RNA quantification data properly.

Searching and indexing expression levels by name is a more tractable and lower ETL cost than resolving specific feature IDs in a sequence annotation database.

[kozbo]

closes #829