Add simple pip install, newick dependency and fix minimal example imports

[scottgigante]

Hi! Thanks for your work on PROSSTT. I've made a couple of changes which I think improve the setup and tutorial experience.

[scottgigante]

I've also replaced the sys.exit in simulation.py with a raise ValueError - it's not very friendly to quit my python interpreter because I did something wrong :)

[scottgigante]

I've also removed the statement about python2 support - https://github.com/soedinglab/prosstt/blob/27c2796d543d94756319a257bf785bc0dfa00799/prosstt/simulation.py#L111 is one example of where Python2 will silently give the wrong results, as Python2 has the following behaviour:

>>> steps = 5
>>> 2 / steps
0

[scottgigante]

I've added another change: tree.Tree.simulate_lineage corrected the genes, not the modules, to the parent - the modules should be corrected instead so they reflect the true gene programs.

[galicae]

Thanks for the suggestions, Scott! Will look into that ASAP and let you know.

[galicae]

Ok, back from vacation and on a real computer again :smile: Thanks for catching these!

[scottgigante]

You're welcome! 😄