Clarify the magnetic geometry.

[mrhardman]

The model used in moment_kinetics is derived in the limit bzed << 1 and rho* << 1, in a magnetic geometry where \vec{b} = bzed \hat{z} + bzeta \hat{\zeta}. As such the equations that we implement do not explicitly feature bzeta ~ 1. This may be confusing when working with relations for the physics at the sheath entrance, which often depend on the very small grazing angle and the sign of bzeta. It may be useful to insert bzeta into the appropriate places in the code to make the origin of the terms more transparent.