Refactor two_point_difference module in jump step for maintainability

[stscijgbot-jp]

Issue JP-3875 was created on JIRA by Melanie Clarke:

Most of the code in the jump step was refactored for maintainability in JP-3697, but code reorganization for the two_point_difference module was deferred to allow some more critical performance improvements for the jump step to proceed.  We will track the remaining code clean up for the jump step here.