Resolve "OTB does not compile with GCC 8.1"

Closes #1706 (closed)

The compilation error arises with gcc 8 which seems to compile more methods in the automatic headers tests.

Those getters where not called in any otb tests or examples, but the error is here since we added this class.

I chose to change the signature of the 2 methods to return a const reference of std::vector instead of a pointer. I think that we don't need to maintain backward compatibility here because as I said the 2 methods could not compile in client code.

I'll add this to the list of changes which break backward compatibility in OTB 7 (next major release?).

I've fixed also 2 warnings related to itk::Exception definitions in OTB test (catching polymorphic type by value)

There are likely other warnings to fix with gcc 8.x.