update to include map_samples

[petrelharp]

Changes recently include:

• added a map_samples argument to simplify to tell you what order the resulting samples are in, because it's no long guarenteed to be like you asked for

• added a optional argument sequence_length to load_tables which makes it possible to specify a tree sequence with zero edges

[ashander]

For reference

• map_samples was changed to map_nodes in #274 from jeromekelleher/final-simplify changed
• sequence_length still good, (added in #272 from jeromekelleher/zero-edge-ts)