Restructure CPP code in to different files

[lukesonnet]

All of the CPP is in one file. I'm going to split it in to different files:

• KRLS functions (functions, gradients, hessians)
• KRLogit functions (...)
• Kernel fucntions (kern_gauss...)
• Solvers (solve_for_d_ls)
• Inference (pwmfx)
• Helper functions (trace_mat, mult_diag, euc_dist)