Document the fact that we are using the E field in uvbeam in a nontstandard way

[dannyjacobs]

We are measuring the polarization of the beam using ENU locked coordinates. For v1.0 we are using pyuvbeam's electric field az_za which projects the E field onto theta, phi coordinate. We will be using Etheta to store E_North and Ephi is E_West. This is a dirty hack.

TODO:

• Actually do this in the current version
• Document the fact
• Propose a PR to pyuvbeam to handle ENU geometry natively (not in V1.0)

[dannyjacobs]

We don't actually use pyuvbeam to write ECHO beams yet. Just use it for reading in CST beams. We compare in power, so no coordinate issues in the short term.