Support for Combined and String features in trees

[vigsterkr]

Currently all the tree implementations, C4.5, CHAID, CART, requires the input feature to be in CDenseFeatures<float64_t>, which is... well not that optimal to say the least. :)

it'd be great that we at least add support for:

• http://shogun.ml/api/latest/singletonshogun_1_1CStringFeatures.html
• http://shogun.ml/api/latest/classshogun_1_1CCombinedFeatures.html

but as well would be great that we use not support float64_t features for DenseFeatures.

[karlnapf]

See #3313 for an example for such a patch

[djokester]

Can I take this up?

[karlnapf]

Yeah do it !

[MikeLing]

Hi @djokester , are you working on it right now? I would like to add this feature to kd-tree and cover-tree if it's ok due to I'm working on the KNN right now. However, I'm totally ok to forget this issue if you had already have some progress on it. Thank you :)