TODO
 - mean/cov: masking
 - lik: gauss,erf
 - inf: ep,la,fitc
 - maybe return only derivatives in ep

2014-03-03
- likErf

2014-03-02
- memoisation bug fixed

2014-01-20
- added memoisation to speed up sq_dist

2014-01-13
 - hyper.py containing the hyperparameter iterator
   common to mean/covariance/likelihood
 - gp prediction
 - likelihood.py: gauss,erf started

2014-01-07
 - gp prediction
 - mean.py: pow+poly
 - covariance.py: rq+matern+gabor, improved sq_dist

2014-01-06
 - inf/gp structure + tests

2013-12-18
 - mean/covariance structure