change n_neighb to num_dp_expected_in_curve

[teese]

The n_neighb is a key parameter for the automatic judgement of curves, but is really difficult to explain.

At some stage, it might make sense to use a value that is easy to explain in the settings file: num_dp_expected_in_curve (number of datapoints expected in curve) or even better, num_dp_expected_at_inflection (number of datapoints expected at inflection)

If we don't want to change the code, n_neighb could simply be half this value: n_neighb = num_dp_expected_at_inflection / 2