Add labels and use equal aspect ratio for axes in plots of radiation pattern in tutorial

[oskooi]

Applies two changes to the axes in the plots of the radiation pattern in polar and 3D coordinates in Tutorial/Radiation Pattern of a Disc in Cylindrical Coordinates:

1. Add missing labels.
2. Use an equal aspect ratio for the xyz axes in 3D rather than the default 4:4:3.

[stevengj]

black wants double quotes in Python strings.