Create an infrastructure for unit tests

[stscijgbot-hstdp]

Issue HLA-1090 was created on JIRA by Steve Goldman:

To simplify the organization of the code and implement unit test, we need to group drizzlepac modules into separate directories. Each directory will have its own 'tests' dir for unit tests of each '.py' file in the directory.

This ticket will mostly be ensuring that the movement of files does not break the paths for imports of various function.