Beam Coordinates

[lkilcher]

Currently when data is in beam-coordinates, only variables in dat.props['rotate_vars'] that start with 'vel' are rotated to beam coordinates. This is because this is how most manufacturers handle data internal to their instruments, but also because rotating a 3-D vector into 4-beam or 5-beam coordinate system has mathematical challenges. This issue is mostly a placeholder to think about whether this should be changed (so that all data is in 'beam' coordinates), or whether this should be left alone (I'm leaning this way).