Add support for options which are sets

[ewu63]

Description of feature

Similar to the list of choices, we should also allow for sets. This would be useful for things like monitorVariables in ADflow, where we want the default options to list out all the possible values. The user options would then be checked that it is a subset of the full set. This would also allow for a description of each value in the options table.

One question is whether these values should be ordered. For safety under parallel execution, we probably want to cast the final set (after error checking) into a sorted list, in case the solvers expect a list to iterate over.