Rename "Starting Temperature" to "Ambient Starting Temperature"

[StevilKnevil]

Used Ta to represent ambient "starting temperature" (rather than Ts for "starting temperature") to avoid confusion with the variables in the "Model Details & Tuning Algorithm" sections, where Ta is "ambient temperature" and Ts is "sensor temperature".

[tombrazier]

Avoiding that confusion with Ts and sensor temperature is a very good idea. Thanks for pointing it out.

Re-reading the procedure, the value Ts (or as you suggest, Ta) has two different roles. In the sensor responsiveness calculation it is the temperature the hotend was at when the calibration began, i.e. T(t) at time t=0. In the ambient transfer coefficient calculation it is properly the ambient temperature, Ta. Since the test is constructed to make these two values the same, I think Ta is probably the best name but it may leave some confused as to why ambient temperature is used in calculating sensor responsiveness. I like the wording "ambient starting temperature", it helps to defuse this latter confusion.

[tombrazier]

@thinkyhead @thisiskeithb I like this PR.

[thisiskeithb]

Works for me, but I don’t have merge powers 🙂

[tombrazier]

But you do have label powers.

[tombrazier]

Why delete and close Stevil?

[StevilKnevil]

My bad! I renamed the source branch to something more sensible, but that resulted in a closed PR! I've reinstated :)

[tombrazier]

Ah, well at least it wasn't something I said then :)