Add interface for constraints on parameters

[mparno]

Unlike the MonotoneComponent class, the RectifiedMultivariateExpansion class requires some of the expansion parameters to be positive in order for the resulting map to be monotone. We currently don't have a good way of defining or imposing these bounds during map training. I propose we add a ParameterBounds() function to the ConditionalMapBase class that returns any lower or upper bounds on the map parameters that are needed to ensure monotonicity. The default behavior could be [-inf,inf] indicating that no bounds are necessary.

[dannys4]

Should be easy to add this to TrainOptions (not considering the bindings). See here and then the SetupOptimization function in TrainMap.cpp