GRHD: add toggle to allow AV to sense based on the log10 of the variable...

GRHD: add toggle to allow AV to sense based on the log10 of the variable (!190)

Adds a new parameter to the `GRHD` section:
- `av_sensor_logabs_D`: If `true` apply the AV sensor to `log(|D|)`, otherwise use `D`, where `D` is the state variable for the rest-mass density (including the volume factor).