vspeed: use unit conversion from utilities module.
tests: fix tests broken by 744a977a and 40f68155.
parameters: fix speedbrake broken in 744a977a.

We need to find a better way of handling per-frame/family/series code in
these nodes that require different routines.
vspeed: various improvements and fixes.

- Initial support for engine series.
- Fix converting of units to match the lookup tables.
- Add additional lookup tables.
- Improve documentation of functions.
- Simplify and document test cases.