Rename iterations_per_second to physics_ticks_per_second

[Calinou]

This makes it clearer that this property is only about physics FPS, not rendering FPS.

The physics_fps project setting was also renamed to physics_ticks_per_second for consistency.

See #16863 (comment).

[lawnjelly]

On the other hand it could be an opportunity to replace the word frame with tick or iteration for 4.x when referring to physics, and standardising terminology.

https://en.wikipedia.org/wiki/Film_frame

The use of the term 'frame' can be, and is, a constant source of confusion (both for beginners, and in terms of explaining concepts) in that the word 'frame' is more normally associated with a rendered frame. It has been bastardised by some to refer to a physics iteration. Whereas a physics tick / iteration need not bear any resemblance to a rendered frame, and indeed explicitly doesn't in Godot. So we end up having to explain that a 'frame' is not a 'frame'...

A physics frame would more often be a physics frame of reference, which is completely different:
https://lmgtfy.com/?q=physics+frame

Without this distinction you have to qualify the term whenever used, and it becomes 'render frame' or 'physics frame' which is far more verbose.

[Calinou]

@lawnjelly I like the idea of using the term "tick" consistently, but I'll wait for others to approve first. "tick" in particular is a popular term in game netcode to refer to update frequency. It makes sense to use the same term for physics too.

[aaronfranke]

I like the idea of changing physics "frames" to another terminology. What about calling this property physics_tickrate?

Side note: ideally, try to avoid referencing the issue in the commit itself, it causes GitHub to post many messages in the issue:

[Calinou]

If we go with the tick terminology, should we rename iterations_per_second to physics_ticks_per_second, physics_tickrate or physics_tick_rate?

[Calinou]

I've remade this pull request and tested it again after discussing it with @pouleyKetchoupp.

[pouleyKetchoupp]

Looks great!

[akien-mga]

Thanks!