Factored out a function "get_likelihood_value" that can be called from different
places.
1 It is called by infer_mean_fast which performa a binary search.
2 It is called by "get_likelihood_function" that wants all the values on the
curve (up to some precision).

This refactoring was necessary (prevent copying of code) when the binary
search was implemented since there was no function that only gave back
single values, we only had a function giving back the whole set of
values from the curve.