Two new checks:
- Added a check to see if Z1/Z2 aren't too low _compared to the
  waterlevel_ instead of NAP (MET_Z_TOO_LOW_BELOW_WATER).

- Added a check to see if a measurement point is not too far away from
  the line between the 22 codes (MET_DISTANCE_TO_MIDLINE).

Fixes #30. Fixes #59.
Preparing release 1.22
Back to development: 1.23