Implement mutual information on `inferenceql.inference.gpm.ensemble.Ensemble`

[Andrew Bolton]

The following mutual information query is implemented in a demo but not in the master branch.

(def test-query
"SELECT MUTUAL INFORMATION OF VAR BoutAz < 1.0 WITH VAR BoutYaw < 1.0 UNDER %s AS mi
FROM (dummy) VALUES ("dummyval") LIMIT 1")

(strict/query (format test-query "clojure_cat_model") model-for-queries)

Error Thrown:

1. Caused by java.lang.IllegalArgumentException
   No implementation of method: :mutual-info of protocol:
   #'inferenceql.inference.gpm.proto/MutualInfo found for class:
   inferenceql.inference.gpm.ensemble.Ensemble