Stops the normal-distribution histogram example from crashing.

Checklist

• I have included examples or tests
• I have updated the change log
• I am listed in the CONTRIBUTORS file
• I have cleaned up the commit history (use rebase and squash)

Changes proposed in this pull request:

• Make the examples deterministic (fixed seed)
• Don't give zero values to Math.Log (this is probably what caused the failure)
• Actually use some of the parameters in the sampling methods (no behavioural change)

@oxyplot/admins