Adding support for customizing ITK namespace

[blowekamp (Bradley Lowekamp)]

This sounds like a good approach, and is common in many of ITK’s third party libraries.

@jcfr

#define ITK_NAMESPACE itk  // default namespace that can be configured
#define ITK_BEGIN_NAMESPACE  namespace ITK_NAMESPACE {
#define ITK_END_NAMESPACE }

Is there a known motivation for ITK_BEGIN_NAMESPACE and ITK_END_NAMESPACE? It seems like it may obscure things, and inhibit reality some.

Also most of the test are outside of the itk name space, so including “ITK_NAMESPACE” as a prefix is going to get rather verbose quickly.