Improve axis labels in documentation

[jmbhughes]

From Craig:

OK, Sarah and I discussed last week calling those coordinates $x_{im}$ and $y_{im}$ in publications. We thought about $x_P$ and $y_P$ (P > for PUNCH), but P was too similar to the common label for a point P; $x_{PUNCH}$ is cumbersome in a publication, so $x_{im}$ it is.

I’m a little leery of using the default headers since each coordinate only corresponds to longitude/latitude along the corresponding axis (as can be seen with the curved fiducials in the reference image), and PUNCH coordinates differ from the traditional definitions of Solar-X and Solar-Y (which are gnomonic-projection coordinates, i.e. in the tangent plane rather than the az-eq projection).

So how do you both feel about using “PUNCH-X” and “PUNCH-Y” in the headers, with a data guide explanation that PUNCH-X is also called $x_{im}$ and is the projection coordinate in the standard PUNCH projection?