Refactor error handling/logging in coregister_dem.py

[truth-quark]

The coregister_dem module uses the "old" style pattern of calling py_gamma repetitively with return code checks and logging.

Refactoring with the error handler/logging decorator ought to remove a few hundred lines of code, like this example: https://github.com/GeoscienceAustralia/gamma_insar/pull/140

No unit tests exist at present, could potentially add something basic for now and fill the rest later?

[sixy6e]

No unit tests exist at present, could potentially add something basic for now and fill the rest later?

Yes, Initially something basic and expand upon later.

[truth-quark]

No unit tests exist at present, could potentially add something basic for now and fill the rest later?

Yes, Initially something basic and expand upon later.

@mwheeler we have to use mocks a lot to get around the gamma interface. tests/test_process_ifg.py has some examples of some basic tests since a lot of the work goes on externally.

[mwheeler]

Code reviewed & merged. Unit testing has it's own ticket w/ more detail at #149