provide method for getting the nodes that correspond to a set of individuals

Proposal: this should be like:

def individual_nodes(inds):
  out = np.repeat(-1, len(inds) * 2).reshape((len(inds), 2))
  for k, ind in enumerate(inds):
    out[k, :] = self.individual(ind).nodes

so returns an array with one row per individual that can be flattened to get just a flat list of the nodes corresponding to the genomes of the individuals whose IDs are inds. And, should insert tskit.NULL as appropriate in the array when individuals are of mixed ploidy.

(moved from https://github.com/tskit-dev/pyslim/issues/39)

[petrelharp]

Sure, we can forget about this. And this is plenty fast even for big tree sequences. I do think that we should streamline working with individuals generally - eg to avoid the error where one passes indiviual IDs to the ‘samples’ argument of simplify - but this is maybe better done by adding an individuals argument to various methods.

[jeromekelleher]

Can you add it to the 0.2.0 milestone so please if you’d like it in?