Calculations for relative probability plots, to assist with assessing the influence of explanatory variables in a model specification. E.g. With all other explanatory variables held at their median value, assess the probability of an alternative with a variable interest at its 5th perentile value and 95th percentile value, and then use the delta in probability as a measure of variable influence.

Added as a SimulationChoiceModel method, so that the relative probabilities can be calculated for any model. Also added a method for calculating probability of a single alternative (given existing alternatives) with user-speficied characteristics. These methods could also be framed in terms of utilities rather than probabilities, e.g. for relative utility plots, but utilities are not currently easily exposed.