Proposal: replace `Running` with a `Speed` property

[daveyostcom]

Proposal:

double Speed property (replaces bool Running)

• Setting Speed to 0 pauses Now.
• Setting Speed > 0 causes Now to advance at the given speed.
• If Speed > 1 and !FutureAllowed, Speed changes to 1 when necessary to ensure that Now ≤ DateTime.Now.
• constraint: Speed ≥ 0.
• default 1

bool FutureAllowed property (new)

• true – allows Now > DateTime.Now.
• false – Now is always ≤ DateTime.Now; Speed is lowered to 1 as necessary.
• default false