drop treat-sim.distributions and replace with distributions from `sim-tools`

[TomMonks]

At the moment treat-sim has its own distributions module. There are no issues, but maintenance effort would be reduced if this was replaced with the same distributions imported from sim-tools.

This would also allow simple solution to #6 i.e. using a truncated normal distribution.

If changed would need to assess the implications for current testing in treat-sim. This should result in less code to maintain overall.