Indicate stellarLocusFit function as semi-private

Prepend the stellarLocusFit function with "_" to indicate that it is
a semi-private function that is unlikely to be used outside of this
module's StellarLocusFitAction.  If, sometime down the road, this is
deemed generally useful, it should be moved to a more generic "utils"
location.

python/lsst/analysis/tools/actions/keyedData/stellarLocusFit.py

@@ -34,7 +34,7 @@
 from ...math import sigmaMad
 
 
-def stellarLocusFit(xs, ys, mags, paramDict):
+def _stellarLocusFit(xs, ys, mags, paramDict):
     """Make a fit to the stellar locus.
 
     Parameters

tests/test_plotUtils.py

@@ -22,7 +22,7 @@
 
 import lsst.utils.tests
 import numpy as np
-from lsst.analysis.tools.actions.keyedData.stellarLocusFit import perpDistance, stellarLocusFit
+from lsst.analysis.tools.actions.keyedData.stellarLocusFit import _stellarLocusFit, perpDistance
 from lsst.analysis.tools.actions.plot.plotUtils import shorten_list
 
 
@@ -49,9 +49,9 @@ def testFitLine(self):
             "nSigmaToClip2": 5.0,
             "minObjectForFit": 3,
         }
-        paramsOut = stellarLocusFit(xs, ys, mags, testParams)
+        paramsOut = _stellarLocusFit(xs, ys, mags, testParams)
 
-        # stellarLocusFit performs two iterations of fitting and also
+        # _stellarLocusFit performs two iterations of fitting and also
         # calculates the perpendicular gradient to the fit line and
         # the points of intersection between the box and the fit
         # line. Test that these are returning what is expected.