Version 2.1 beta 3 -- live unit switching

HUGE thanks to @Millardiang, who has managed to figure out how to implement live unit switching for EVERYTHING in the chart, including forecast, charts, etc. etc. A very handy feature!

The switching is off by default. You can enable it by setting unit_switching = 1. Please note this option does significantly increase processing requirements!

Beta 3 also has associated bug fixes, most notably a fix for long processing times for stations missing optional measurements.

I also split development off into a separate branch. From now on, master contains only stable releases, and everything else is on development, including beta releases.