Define a generic directory type

[vyasr]

Feature description

The Job and Project classes are both handles on different types of directories. signac's data model encodes a specific data layout that implies a relationship between the two, but there are also various pieces of common functionality that could be isolated into a common class. Moreover, any attempt to generalize the data layouts signac supports would require some concept of operating on an arbitrary directory within some directory tree structure.

Proposed solution

We should implement a common signac.Directory base class that handles the basic common functionality of a single directory. The initial starting point would simply be extracting common functions out of the Job and Project. Further inspiration may be found in the signac-2 prototype. If #96 is implemented, a directory could also be identified by a single identifier.