Add meta example facility for director classes

[karlnapf]

This allows testing #3894 #3893 #3892

[sorig]

Can you write a bit more context on this? How does the language need to be extended?

[karlnapf]

If we could extend the meta language so that it allows for overloading of a method of an object, then we could test director classes functionality without extending the current test infrastructure.

Also, these examples should illustrate why Director classes are cool.

The only problem is that we cannot really write code in the meta language, only Shogun API.
I think one of the nicest things to overload is a method of CKernel, so maybe we go for CKernel::kernel and explicitly return something like the product of the indices (we would need product of basic types for that :/ )? The example then could call get_kernel_matrix, which is C++ code which then runs the kernel code from inside. Maybe there is a better example, it has to be catchy.

Steps

• Read on Python's SWIG director classes here
• The first step would be to compile Shogun with Director classes enabled -DUSE_SWIG_DIRECTORS=On
• then try to overload a method in an explicitly written Python example, see if it works
• Then do it for other languages (including C++)
• Think how we can modify the grammar.