Reftran --reference-frame argument to be optional (adopt existing project datum and epoch)

[jhaasdyk-au]

Brief description
Enhance reftran to make --reference-frame argument optional.
i.e. adopt the existing project datum (and epoch) previously set during import

Related issue(s)
None

Background
Currently refran requires the argument --reference-frame, else exits and reports "- Nothing to do - reference frame was not supplied.”

However, the user may want to run reftran simply to align all stn and msr to the current project datum.
There should be no requirement to (re) specify --reference-frame

Related to discussions held 15/06/2023 [Roger Fraser, Joel Haasdyk, Alex Woods], and recent emails, investigating:

• the setting of project datum at the start of import [and use-cases where a default value 'GDA2020' is adopted for this purpose]
• the role of project datum during import of stn and msr files [specifically where stn and msr do not define their own datum]
• the flow of project datum through import, refran and adjust

Basic requirements
refran called without the argument --reference-frame executes using the existing project datum (and epoch) as the target for all transformations.

Priority
Priority 3 (low)