UniformFoMMetric summary metric

[rmandelb]

This draft PR is for the new summary metric (based on this notebook). The work is not yet complete, with 2 TODOs listed at the top and one in the middle (comment beginning with ###). These include:

• Figuring out how to constrain it to all exposures up to a particular year (self-consistently for the metric on which this is based and when calling the 3x2pt FoM metric in run()).
• Figuring out how to put in the constraints on dust, DDFs, twilight exposures (self-consistently for the metric on which this is based and when calling the 3x2pt FoM metric in run()).
• Actually calling the 3x2pt FoM metric -- I wasn't quite sure how to do that from within MAF.

I plan to work on it more in the morning, but there's already a lot there so I wanted to get some eyes on it. Advice on the above is welcome. (With these things still to do, I haven't been able to run this yet, so I am sure there are bugs.)

[rmandelb]

Boris has made great progress towards reformulating this metric and incorporating it into his code in #397 . We have a few more checks to do once NERSC is back up tomorrow, but I'm confident that his approach is going to work, so I'm closing this draft PR. Thanks, Peter and Lynne, for all the advice about how to formulate these new summary metrics.