Defer matplotlib loading until we need it

Only one function in interactive.py uses matplotlib so load
matplotlib when that function is called.

python/lsst/meas/modelfit/display/interactive.py

@@ -110,6 +110,7 @@ def fit(self, outRecord):
     def plotDistribution(self, *records):
         """Plot a representation of the posterior distribution from a ModelFitRecord.
         """
+        import matplotlib
         recordId = records[0].getId()
         figure = matplotlib.pyplot.figure(recordId, figsize=(10, 10))
         data = {}