Add TimePosition double constuctor in seconds

[jamierpond]

Basically allows constructions of TimeRanges with branced loops, and implicit casting of doubles to TimePositions.

[drowaudio]

I know it seems annoying but I deliberately didn't want to be able to construct a TimePosition from a double. The whole point is that a TimePosition isn't in "seconds" so constructing one from a double is ambiguous. Is the double minutes, hours, frames etc. That's the whole point of the explicit fromSeconds function.

If you're using compile-time constants, you can use the chrono literals to construct them e.g. 1s, 2ms etc.

[drowaudio]

And can't you construct a BarsAndBeats using aggregate initialisation like this: BarsAndBeats ({ numBars })?

[jamierpond]

Hmmm... that's fair. I thought this was probably the case.

BarsAndBeats needed a more explicit constructor since now it's ambiguous whether it's constructing that or a TimeRange.