Sensor: Extract gravity vector from current attitude

**Requester:** @NArmistead 

**Describe the solution you'd like**
In order to get useful data from our accelerometer for sensor fusion, we need to subtract gravity from our readings. Use our current attitude to get the XYZ vector of gravity. 

**Describe alternatives you've considered**
None

**Additional context**
N/A

**Points of Contact**
Sensor stuff: @NArmistead 