Move all file handling out of ArchivesSpaceClient

[helrond]

Is your feature request related to a problem? Please describe.
Handling for filepaths pointing to ArchivesSpace configs is split between the main DACSspace and ArchivesSpaceClient classes. This creates additional complexity in tests and means that a number of assumptions are built into the use of the library which may not be true in the future.

Describe the solution you'd like
Put all filepath handling in the __init__ method of the main DACSspace class, which would keep all of that stuff in one place and allow us to make the client totally independent of the underlying filesystem.

Describe alternatives you've considered
Leave as-is. We decided this was not the best option.