TimerDeltaSigma

public struct TimerDeltaSigma

Usage examples:

Sample usage:

   func demo_TimerDeltaSigma() {
       var tds = TimerDeltaSigma()
       for _ in 0 ..< 10 {
           tds.printElapsedTimes()
       }
       print()
       for _ in 0 ..< 5 {
           tds.printElapsedTimes(.ms)
       }
       print()
       for _ in 0 ..< 5 {
           tds.printElapsedTimes(.s)
       }
       print()
   }
```
Sample  output:

```
    Δ 0.000000, Σ 0.000000 s
    Δ 0.000082, Σ 0.000082 s
    ...
    Δ 0.000022, Σ 0.000260 s
    Δ 0.000021, Σ 0.000281 s

    Δ 0.000, Σ 0.000 s
    ...
    Δ 0.000, Σ 0.000 s

    Δ 0, Σ 0 s
    ...
    Δ 0, Σ 0 s
```