PERF: Minimize redundant function calls

The highest percentage of time in the MattesMutualInformation
filter was spent evaluating if specialized work needed to
be done for displacment fields.

In a minimalistic test case, prior to this change the registration
took 846 seconds, after this change, registration took 309 seconds.

bin/BRAINSFit   \
  "--costMetric" "MMI"  \
  "--failureExitCode" "-1"   \
  "--numberOfIterations" "1"  \
  "--numberOfHistogramBins" "50"  \
  "--splineGridSize" "7,5,6"  \
  "--numberOfSamples" "14000"   \
  "--minimumStepLength" "0.1"  \
  "--initializeTransformMode" "useCenterOfHeadAlign"  \
  "--transformType" "BSpline"  \
  "--fixedVolume"  "BRAINSTools-build/ExternalData/TestData/test.nii.gz"  \
  "--movingVolume" "BRAINSTools-build/ExternalData/TestData/rotation.rescale.rigid.nii.gz"  \
  "--outputVolume" "BRAINSTools-build/BRAINSFit/TestSuite/BRAINSFitTest_BSplineScaleRotationRescaleHeadMasks.test.nii.gz"  \
  "--outputTransform" "BRAINSTools-build/BRAINSFit/TestSuite/BRAINSFitTest_BSplineScaleRotationRescaleHeadMasks.h5"  \
  "--maxBSplineDisplacement" "7.3"

Change-Id: I3782e78cae99acfe577a40444882688ae5b76472
PERF: Pull loop termination constants out of loop

When evaluating loops, loop termination criteria that
are accessed via member functions should not be
used at every iteration.  Common static loop iteration
termination values are stored locally and reused.

Change-Id: Ia847f411640cce6921b28ac2a2da448c40ebe24c