Add new NDVI filter

As a result of the way frei0r.hpp must work, then paramColorMap is redundant. However, there is not a super clean and simple way of obtaining the value like the other param types. Rather, you will have to call get_param_value() or do something like static_cast<std::string*>(param_ptrs[param_index]). The problem then becomes, what is the param_index? You can use frei0r::s_params.size() - 1 after the call to register_param() to save the index.