if IterationSetup is provided, and InvocationCount and UnrollFactor are not, run benchmark once per iteration to avoid user confusion

[adamsitnik]

fixes #730

Example:

public class FileSample
{
    private readonly string filePath = Path.GetTempFileName();

    [IterationSetup]
    public void Create()
    {
        using (File.Create(filePath)) { }
    }

    [Benchmark]
    public void Remove() => File.Delete(filePath);
}

/cc @stephentoub @nietras

[adamsitnik]

@AndreyAkinshin The histogram for my example looks really nice!

-------------------- Histogram --------------------
[0.979 ms ; 1.701 ms) | @@@
[1.701 ms ; 2.227 ms) | @@@@
[2.227 ms ; 2.573 ms) |
[2.573 ms ; 3.064 ms) | @@@@@@@@@@@@@
[3.064 ms ; 3.590 ms) | @@@@@@@@@@@@@@@@@@@@@@@@
[3.590 ms ; 4.186 ms) | @@@@@@
[4.186 ms ; 4.776 ms) | @@@@
[4.776 ms ; 5.470 ms) | @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
[5.470 ms ; 5.989 ms) | @@@@@
[5.989 ms ; 6.424 ms) | @@
[6.424 ms ; 6.689 ms) |
[6.689 ms ; 7.215 ms) | @@@@
[7.215 ms ; 7.826 ms) | @
---------------------------------------------------

And I got nice multimodal warning:

MultimodalDistribution
  FileSample.Remove: InvocationCount=1, UnrollFactor=1 -> It seems that the distribution is bimodal (mValue = 3,86666666666667)

[AndreyAkinshin]

LGTM