Replace Normal distribution with Normal with lower bound 0.

[TomMonks]

As per Nelson (2013), the examination activity uses a Normal distribution with mean 16. During experimentation this mean may be lowered. In some cases a negative value may be sampled. The normal distribution should be replaced with a normal distribution with a lower truncation at zero.

Reference of distributions used is here: https://pythonhealthdatascience.github.io/stars-simpy-example-docs/content/03_stress/03_data.html

Note: that sim-tools implements a normal distribution that can resample if sample < 0. All dists could be replaced with sim-tool dependency.

[TomMonks]

Fixed in version 2.0.0

[TomMonks]

Needs to be updated in documentation.