Calculate Wheel Size from BB Drop and BB height

[jdtoy]

When both the bottom bracket (BB) height and the bottom bracket drop are specified as inputs, calculate the wheel size (outer radius). This wheel size is likely to be more correct, diagrammatically, than currently methods.

Reference on wiki: Points on a Bike

[Bob-GG]

BB Drop is often provided in geo tables with a + or - sign. On the great majority of bikes it's "downward", i.e. BB lies below a line between the front and rear axles.

As per a discussion jdtoy and I had, we propose:

• If unsigned, assume downward
• If -ve, assume downward
• If +ve, assume upward

[jdtoy]

I've implemented a fix for the +/- parsing issue. Using this corrected value to calculate the Outer wheel diameter is still to do.