Include pairtree_prefix logic in id to dirpath and dirpath to id

[rlskoeser]

The get_id_from_dirpath and id_to_dirpath methods in pairtree_path don't seem to have any logic to acocunt for the optional pairtree_prefix file - it would be really helpful to have.

Am I missing something? Is this handled somewhere else?

[benosteen]

This may not be handled elsewhere as you politely suggested! Probably best to include it in the functions themselves...