entropy_search_big_data: fixed bug where representer points would have one
	coordinate too much or less, dividing by exp(costs)
OptSizeChooser: moved more parameters into constructor, found promising
	to local optimization
bigdata_deception: enhanced output
gp_model: added kernels, fixed small bug when using non-normalized kernel
pmin_discretization: better mixing for representer_points
abstract_test: changed finite differences test (more flexible), standard
	covariance function is BigData
ei_test: adapted to changes in abstract test
covar_test: enhanced usage of finite differences test
gp_test: now checking that predicted variance is positive, using finite
	differences test