resolve plotting install infelicities

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