[DEBT] rename .h|.hpp|.cuh|.cu|.cpp files properly in our cuml C++ source base

Currently, they are just all over the place! I propose the following rule-of-thumb:

1. If a header file contains cuda kernels and/or device methods, then it should be a .cuh.
2. Else, it should be a .hpp (if the current header file is already named as a .h, feel free to leave it as it is, in order to minimize the amount of ripple changes this will cause).
3. If a header file is used to declare our C interface, it should be a .h
4. If the header file includes a .cuh, then it should be a .cuh as well
5. If a source includes a .cuh, then it should be a .cu, else it should be a .cpp
6. All source files defining our C interface should be .c

This will certainly help us in reducing compilation time as well (if we end up cleaning some of the .cu files and renaming them to .cpp).