Added pep.py -- Sparse Gaussian processes using Power Expectation Propagation
Added pep.py -- Sparse Gaussian processes using Power Expectation Propagation

This allows interpolation between FITC (EP or alpha = 1), and Titsias's variational (VarDTC, VFE when alpha = 0).

Reference: A Unifying Framework for Sparse Gaussian Process Approximation using Power Expectation Propagation https://arxiv.org/abs/1605.07066
befelix/GPy,mikecroucher/GPy,befelix/GPy,SheffieldML/GPy,dhhjx880713/GPy,dhhjx880713/GPy,esiivola/GPYgradients,befelix/GPy,befelix/GPy,mikecroucher/GPy,mikecroucher/GPy,SheffieldML/GPy,ysekky/GPy,ysekky/GPy,mikecroucher/GPy,esiivola/GPYgradients,esiivola/GPYgradients,esiivola/GPYgradients,SheffieldML/GPy,dhhjx880713/GPy,dhhjx880713/GPy,SheffieldML/GPy,ysekky/GPy,ysekky/GPy