RunPlanVector potential changes/enhancements

[ptheywood]

While testing / working with RunPlans and RunPlanVectors several potential improvements came up, but to avoid blocking bugfixes they're getting their own issue.

• RunPlanVector::setPropertyUniformDistribution has a potentially misleading/ambiguos name, which is similar to setPropertyUniformRandom, which uses a Unform distribution. It should probably be renamed. It's more of a LERP imo, but there may be a more canonical name for this for similar use-cases.
• operator== would be useful to compare RunPlan instances (and potentially RunPlanVectors) against one another. greatly simplifying some tests
• RunPlanVector::at(size_t index). I.e. expose the vectors .at methods, which is operator[] with bounds checking.

[Robadob]

RunPlanVector privately extends std::vector, so just need using statement added for at. There's already a bunch there.