Remove fitting code from BQ
Add option to use approximation
Update tests
Rerun notebooks
Use approximation for expected squared mean as well
Add periodic example notebook
Remove 'ntry' option
Remove 'fit' method; misc approximation changes
Update gaussian processes version and include matplotlib in requirements.txt
Don't use make_approx_x until x_sc is defined
Increase efficiency of slice sampler
Add tests
Rerun periodic example notebook