convert primary_keys arguments to primary_key

there are several places where we deal with the attribute or argument "primary_keys". Kevin Dangoor properly pointed out that a table has only one primary key, which might be composite and consist of multiple columns, but its still just one primary key.

so wherever an argument "primary_keys" exists, make it "primary_key", and make it take a scalar or a list. convert attributes named "primary_keys" to "primary_key".