NestedSampler abstract class predefine _needs_sensitivity

[chonlei]

Not sure if this is a good practice that in the NestedSampler, self._needs_sensitivity is always needed (e.g. needs_sensitivities() method), but it is never defined in the abstract class. At the moment it is only provided in each subclass. So it works fine but when I was looking through the code I was expecting if the subclass doesn't define that would raise NotImplementedError but maybe not AttributeError...? Unless I've missed it somewhere!

[MichaelClerx]

Yep, sounds about right!