adding command to make nexus files

[SimonGreenhill]

This PR adds a custom command to make a nexus file from the cldf dataset. Run as follows:

# basic nexus
cldfbench abvdoceanic.nexus  --output abvdoutliers.nex

# generate with ascertainment column for overall ascertainment correction.
cldfbench abvdoceanic.nexus --ascertainment overall --output abvdoutliers.asc.nex

# generate with ascertainment column per word for ascertainment correction on a word-by-word basis.
cldfbench abvdoceanic.nexus --ascertainment word --output abvdoutliers.words.nex

Note that this depends on the [NexusMaker](github.com/SimonGreenhill/NexusMaker) library, so you'll need to pip install nexusmaker or re-run setup.py.

[antipodite]

That's cool as , shall I just go ahead and merge it?

[SimonGreenhill]

done :)