Unit tests for non-parametric regression

This is a WIP pull request to add unit tests for the non-parametric regression models ```KernelReg```, ```KernelCensoredReg```, ```SingleIndexModel``` and ```SemiLinear```. Tests will be based on examples given in the documentation for the R ```np``` package.

So far changes are
- Inclusion of two new data sets (documentation of these needs to be improved)
- New test file for the semi-parametric regression classes