Example implementation of Generator base class in the context of Xopt

[roussel-ryan]

See the new branch https://github.com/ChristopherMayes/Xopt/tree/generator_standard for a basic attempt at inhereting from the CAMPA Generator standard in Xopt. Works as advertised incl. in a basic BO example (see here) with minimal changes to the Xopt code. If we want to make the standard available on conda that would further clean up the code (I've just copied and pasted the code into a file).