resolve plotting install infelicities

[samueldmcdermott]

currently, installing namaster is a bit inelegant, since the user needs to know something about their chip architecture if they're using a Mac. We could make this a bit easier by putting in some install scripts for the conda venv and the pip install, though this also opens up the issues of using different package managers.

Alternately, we could drop namaster dependence and switch to pixell, though this requires a bit more of a change to the code

[samueldmcdermott]

(not marking this a bug or priority because we are making this an extra for now, as in #76)