Add virtual descructor to RadiometricIndex abstract base class

Fix undefined behavior (https://stackoverflow.com/a/461224/565840) and warning:

/usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/unique_ptr.h:78:2: warning: delete called on 'otb::Functor::RadiometricIndex<float, float>' that is abstract but has non-virtual destructor [-Wdelete-non-virtual-dtor]