Calibration should perform linear interpolation to level

https://github.com/rainchasers/content/blob/a2664a206411700a76d46ce853ee363c1aff1ee3/internal/river/calibration.go#L16-L21

If a level minimum isn't currently specified it is ignored: e.g. if low and high is specified it is impossible for this code to infer the river is at medium. This should be possible via linear interpolation. 