COMP: itk::Command-derived class used pointless dynamic_cast.

There is a common pattern in the ITK Examples and tests:
Use dynamic_cast to downcast the 'object' parameter to
the 'Execute' method, then ignore the return value.

Given that itk::Command-derived classes are tuned to a
single datatype, unless a programmer has screwed up,
dynamic_cast will always succeed. This means it might as well
be static_cast, especially if the return value is never checked.

So I replaced the dynamic_cast with static_cast where they
occurred in the Execute method of itk::Command derived classes
with static_cast.

Change-Id: I77f0735598ad5657e2a7a6114fee004ab1daf3d9
Merge topic 'itkCommandDynamicCasts'

845069e8 COMP: itk::Command-derived class used pointless dynamic_cast.
BUG: Expand a test to reflect multi-metric registration bug

In this patch the "itkSimpleImageRegistrationTest2" is extented to use
an additional rigid registration stage with a multi-metric that contains
only "one" metric component.
This test is supposed to show a bug in ITKv4 registration framework when
it is used with multi metrics.
This test will be failed by returning an exception because current
ImageRegistrationMethodv4 always assumes that multi metrics have more
than one component when it tries to set the metric sample points.
Also, in the extented test, the multi-stage registration is implemented
in a cascaded way that is different than the methods used in other
existent tests, and it can be a bonus itself.

Change-Id: Ibfa9b5c65794ec5cf369c9aef84bb822eca799f0
BUG: Fix multi metric point sampling bug in ImageRegistrationMethodv4

ITKv4 registration framework always assumes that multi metric has more
than one metric component; however, multi metric should work even if it
has only one component. It makes that easier to generalize the code to
use different metric components by defining only one multi metric object
for that.

Change-Id: Ibf508a8cb223435afda4cf36214a4d5259641759
Merge topic 'multiMetricBug_2'

55266ba7 BUG: Fix multi metric point sampling bug in ImageRegistrationMethodv4
b8fc6183 BUG: Expand a test to reflect multi-metric registration bug
ENH: Add one method to erase a tag into MetaDataDictionary

Change-Id: I3bd9b7c92a71cec85b69180d88a18728a2d892db
Merge topic 'MetaDataDictionary'

09a0bc8e ENH: Add one method to erase a tag into MetaDataDictionary