add labeling functions for axes?

[bbolker]

Some miscellaneous functionality here for

• scientific notation with superscripts (i.e. 10^3, 10^4, etc.)
• latitude/longitude scales

These could be further improved by

• allowing for a LaTeX (intended for tikzDevice, e.g. $10^2$) vs plotmath output (e.g. parse(text="10^2"));
• reducing code duplication in lat/long, e.g. generating lat/long scales from a single factory function that determines whether to use E/W or N/S legends
• could further improve lat/long by allowing it to (optionally) express as degrees/minutes/seconds rather than decimal degrees
• make a temperature scale function

Does this functionality exist somewhere else in the gg-verse?