Issue #1103: removed CCombinedKernel::get_first_kernel()

Removed CCombinedKernel::get_first_kernel() by 
- Removing the function in CombinedKernel.h
- Replacing function calls of get_frist_kernel() in other files by get_kernel(0)
 
Did NOT remove get_last_kernel() because the following files will need this function:
- src/shogun/ui/GUIKernel.cpp:		
- src/shogun/ui/SGInterface.cpp:

and I'm not sure what to do for these function calls.