Add CP and CW kernels

[BracketJohn]

DoD

• Have working CP Kernel
• Have working CW Kernel

Tasks (#25)

• Research how gpflow issue does it
• Research how as copies it from gpml
• Create new kernel package
  • Implement CW kernel
  • Implement CP kernel

[BracketJohn]

This was postponed, as it is not necessary to finish this paper/thesis. Might still be very relevant though.