Consume float values from OPTICAL_FLOW mavlink message instead of integers

## Feature request

**Is your feature request related to a problem? Please describe.**
We consume integer fields rather than floats; the precision could be useful

**Describe the solution you'd like**
Use the float fields

**Describe alternatives you've considered**
Not.

**Platform**
[  ] All
[  ] AntennaTracker
[ X ] Copter
[ X ] Plane
[ X ] Rover
[  ] Submarine

