REFAC: Use FunctorImageFilter to rewrite leveling filter, and remove spurious includes of UnaryFunctorImageFilter

ENH: Add another New() implementation enabled for non default construcible TFunctor so as to get a nice warning (thanks to @aregimbeau for the patch)

Verbose libsvm is only enabled when building in Debug

In remote modules for instance, old params such as EmptyParameter
or ComplexInputImageParameter are still in use. We can ensure
that QgisDescriptor support them.

Even if these parameters are deprecated, the generator should at least recognize them and treat them as normal images