Generalize diamond distance wildcard models

[coreyostrove]

There is a new feature being develop that adds support for wildcard models where the weighting is based on the diamond distance between the target operations and the estimated ones. There is no reason this couldn't be generalized to other error metrics, so this feature would add support for some number of additional error metrics beyond diamond distance. It is also the case that cvxpy is sometimes unavailable on certain systems, so this would all use of this technique in those cases as well.

[sserita]

The final commits of PR #267 laid the groundwork for this, although additional options have not yet been implemented.