DOC: Corrected documentation for threshold boundaries.

Previously, the documentation for itkThresholdImageFilter was
incorrect for the ThresholdAbove, ThresholdBelow, and ThresholdBetween
options because they incorrectly stated the handling of pixels in the
image equal to the threshold value itself.  The previous comments
stated that pixels equal to the threshold value will be changed to the
outsideValue.  But according to the code, the true handling of pixels
equal to the threshold value is that they are not affected by the
threshold.  The documentation has now been corrected.

The corresponding test was enhanced to add explicit testing of the
threshold value, and the new tests pass, further indicating that the
new documentation is correct.  The test was also extended to handle
cases where the inputValue is not 0 for generality and to make the
code clearer.

Change-Id: I3fc7f36a532e7b64b03d9591e31057e7d1cc745d
STYLE: Consistency of threadID and threadId

Comparing sources within the V4 registration code
was complicated by different use of the spelling
for local variable identifying the thread identifier.

By making this variable the same accross the toolkit,
code comparisons are made simplier and easier to keep
consistent.

Change-Id: I70bbbd2be9116c6c30472a1c93edd0b41d37b6f6
DOC: Convert seven ImageRegistration examples to ITKv4

Seven registration examples are refactored to use ITKv4 registration
framework. Also, the explanations related to the deprecated features
are remove, and new explanations are added to reflect the new
features.

Change-Id: I5d01ca5be3522d53d64a861a7bef14872e9c3f63