Switch from marginnote to marginpar

[jhuggins]

Currently margin comments use the marginnote package. However, there are problems with this choice:

1. This package is no longer maintained (see the documentation)
2. If margin comments are close together, they will overlap and become unreadable.

The advantage of marginnote is that it can be used inside floats. However, I don't think that benefit outweighs the major problems. If we really wanted to support both, I would suggest adding a separate command like \mfXX (for "margin float") that uses marginnote.