Use float division when converting wheel size

In training mode, calculations of speed using wheel size and RPM from
a speed sensor used integer division. This truncated all wheel
circumferences to an integral number of meters. 2100mm wheels were
treated as 2 meters and 1960mm wheels were treated as 1 meter,
effectively reducing calculated speeds by roughly 5% and 50%
respectively.

Fixes #585
Fixes #651
Fixes #669