Review and refactor orbital module

[mcgarth]

In light of the big changes to the Orbital module in https://github.com/GeoscienceAustralia/PyRate/pull/303, review and refactor the module. Here are some considerations:

• communicate to users when is it worth using the orbital correction (what sensor, size of imaged area etc).
• more testing of the use of scale factor in the DM for network method and refine the default value if necessary.
• refactor the flow of the module and re-use functionality for both independent and network methods where possible (e.g. multi-looking)
• reconsider the need for the application of the offset as the median of the image during the orbital module (pyrate.core.ref_phs_est can be ran before and/or afterwards to have the same impact on the data)