Feature Request: Discrete variable specification/optimization

[roussel-ryan]

There are a number of contexts were discrete variables are required to perform optimization, specifically multi-task optimization at discrete fidelities.

TODO list:

• implement discrete variable specification in VOCS
• add validation to generators that support/doesn't support discrete values
• implement discrete BO algorithms

[roussel-ryan]

Note: this can be partially addressed in BO algorithms by using grid based optimization of the acquisition function