COMP: SimpleITK linkage failure

When building with clang on 10.9 where ITKv4 is a shared library,
a linking error occured for unfound symbols: NumericTraits< OutputPixelType >::Zero
but using the NumericTraits< OutputPixelType >::ZeroValue()
allows linkage to occur properly.

Change-Id: I496a41c102960b60c461adefdccc373b39dabb03
Merge topic 'FixZeroValue'

9ed7059 COMP: SimpleITK linkage failure
BUG: Prevent ObjectFactoryBase from possibly throwing an exception

Coverity produces over 250 errors due to the fact that
ObjectFactoryBase::RegisterInternal() has the structural capability to
throw an uncaught exception, if m_RegisteredFactories is non-empty.

However, as currently implemented, when ObjectFactoryBase::
RegisterInternal() is called, it is logically impossible for
m_RegisteredFactories to be non-empty. Verifying this precondition as a
check against future bugs is still good idea, but errors will now only be
produced in debug mode. In all build configurations, m_RegisteredFactories
will be cleared (which should be a small, constant-time overhead), to
guarantee consistent behavior.

Change-Id: Ia5890a323a7e8d5ed6b44195158a7a05f2c5df3e
Merge topic 'ObjectFactoryException'

302d53c BUG: Prevent ObjectFactoryBase from possibly throwing an exception