Make DiscreteVectorParam nicer if all values are character

[mb706]

For most Vector parameters, the values they accept are (atomic) vectors of values in their feasible set. An exception of note is the "discretevector", which uses lists of its feasible values. This is obviously necessary when the discrete values are complicated types. Often enough, however, the possible values of a "discretevector" are all just character(1), in which case it would be very useful if one could use character vectors as values of these discretevector parameters.

I suggest a use.atomic flag (or similar) for makeDiscreteVector[Learner]Param, which

1. requires all feasible values to be character (or possibly all numeric, all integer etc.).
2. uses character vectors (or possibly other types) as values instead of lists.

I don't know ParamHelpers internals enough to know what changes 2. would require, one obvious place is constraintsOkParam.