Add exposure length checking

Was not previously checking that the user selected exposures
with equal exposure length. Appears in two places so added it
as a function.

python/lsst/cp/pipe/makeBrighterFatterKernel.py

@@ -410,6 +410,7 @@ def run(self, dataRef, visitPairs):
             dataRef.dataId['visit'] = v2
             exp2 = self.isr.runDataRef(dataRef).exposure
             del dataRef.dataId['visit']
+            self._checkExpLengthEqual(exp1, exp2, v1, v2)
 
             self.log.info('Preparing images for cross-correlation calculation for detector %s' % detNum)
             # note the shape of these returns depends on level
@@ -749,6 +750,35 @@ def estimateGains(self, dataRef, visitPairs):
             gains[ampName] = 1.0/slopeToUse
         return gains, nomGains
 
+    @staticmethod
+    def _checkExpLengthEqual(exp1, exp2, v1=None, v2=None):
+        """Check the exposure lengths of two exposures are equal.
+
+        Parameters:
+        -----------
+        exp1 : `lsst.afw.image.exposure.ExposureF`
+            First exposure to check
+        exp2 : `lsst.afw.image.exposure.ExposureF`
+            Second exposure to check
+        v1 : `int` or `str`, optional
+            First visit of the visit pair
+        v2 : `int` or `str`, optional
+            Second visit of the visit pair
+
+        Raises:
+        -------
+        RuntimeError
+            Raised if the exposure lengths of the two expsoures are not equal
+        """
+        expTime1 = exp1.getInfo().getVisitInfo().getExposureTime()
+        expTime2 = exp2.getInfo().getVisitInfo().getExposureTime()
+        if expTime1 != expTime2:
+            msg = "Exposure lengths for visit pairs must be equal. " + \
+                  "Found %s and %s" % (expTime1, expTime2)
+            if v1 and v2:
+                msg += " for visit pair %s, %s" % (v1, v2)
+            raise RuntimeError(msg)
+
     def _calcMeansAndVars(self, dataRef, v1, v2):
         """Calculate the means, vars, covars, and retieve the nominal gains,
         for each amp in each detector.
@@ -792,6 +822,7 @@ def _calcMeansAndVars(self, dataRef, v1, v2):
         exp2 = self.isr.runDataRef(dataRef).exposure
         del dataRef.dataId['visit']
         exps = [exp1, exp2]
+        self._checkExpLengthEqual(exp1, exp2, v1, v2)
 
         detector = exps[0].getDetector()  # note we lose the detector in the next line as they belong to exp
         ims = [self._convertImagelikeToFloatImage(exp) for exp in exps]