2.x: Support alternative unit display

[bradh]

What is the use case for your proposed feature? Please describe.
When we display values (e.g. with getDisplayableValue()) we are getting whatever made sense at the time converted to a string. Sometimes it has unit / symbols (e.g. for degrees or pixels). It would be useful to be able to have the display in something user selectable (e.g. latitude in decimal degrees, degrees-minutes+decimal, degrees-minutes-seconds+decimal, etc; height in metres or feet). Note that context matters - I might want the pitch angle in decimal degrees, but the position in DMS.

Describe the solution you'd like
Probably we need some convenience API to generate them, and a configuration object to be able to select what kind of display is needed which would get passed in to getDisplayableValue(DisplayUnitConfiguration config) or similar.

Its a lot of editing, although not particularly difficult. Will just need to grind through everything we have implemented.

Describe alternatives you've considered
A global configuration would also be possible, but might get ugly in a shared server implementation.

Additional context
Because there are potentially flow-through impacts on shared code (e.g. ST 0603, or ST 1010), this would be better done for 2.x.