Move & refactor Gamma decorator to py_gamma_ga module

[truth-quark]

Relates to the Gamma error handler/logging decorator here: https://github.com/GeoscienceAustralia/gamma_insar/blob/pygamma_workflow/insar/process_ifg.py#L27-L56

A generic decorator is needed for other modules to import, rather than creating multiple equivalent/similar decorators. The simplest option is to:

• move the process_ifg.py "experimental" decorator across to py_gamma_ga.py
• add new arg to specify type of exception to be thrown
• fix module imports
• provide a better name (decorator to ? Something along the lines of auto logging/exceptions)
• potentially add extra tests for py_gamma_ga.py

[truth-quark]

Fixed with https://github.com/GeoscienceAustralia/gamma_insar/pull/139.

Github isn't auto closing linked issues for some reason.