get_neighbors() should not be implemented for algorithms that cant use it

[derek-saal]

Description

get_neighbors() is under AlgoBase() when it can't be used by all those that extend AlgoBase()

Expected Results

Consider removing this from base and implementing it where relevant.

[NicolasHug]

It can be used by any algorithm that uses a similarity matrix and I feel that it's not worth having another base class just for that.

But it's true that we could have a better error message in case the 'self.sim' attribute does not exist. Feel free to submit a PR!