Model parsing can fail on some OSX instances unless lxml package installed

[sclamons]

BeautifulSoup, which is used by Bioscrape's model parser, requires a tree parser to work. It can use Python's default HTML parser, but some OSX installations use an old enough core Python that BeautifulSoup refuses to use it, resulting in an error where BeautifulSoup claims it can't find a TreeParser. This can be fixed manually by installing the "lxml" package (pip install lxml).

Minimal fix -- document this problem in installation notes.

More complete fix -- make sure there's a compatible tree parser during installation, and if there isn't, install one.