Hierarchical DIP support

This adds support for a new form of structMap, the logical structMap.

Directories created via SIP arrange can now be assigned a level of description label, in addition to a name. If directories with levels of description exist when the METS is created, then a logical structMap is created alongside the physical structMap. The structure of the logical structMap is identical, but the `type` value for directories is now the assigned level of description instead of `Directory`. The logical structMap can be used to assign levels of description to records on import to access systems like AtoM.

Major changes in this branch:

* AtoM upload configuration has been refactored, and now now form-based; the user no longer directly writes an argument string.
* If an AtoM system has been configured for upload, then the levels of description can be fetched from AtoM for use in SIP arrange.
* The hierarchy of levels of description can be configured in Archivematica. (For instance, in custom LOD hierarchies, to control what levels are higher than others.)
* Levels of description can be assigned to directories when they are created in SIP arrange.
* Logical structMap creation in createMETS2.