Cloud base < 100 m

### What problem does your feature request solve?

For aviation purposes it is important to know when the cloud base is less than 100 m.

### Describe the solution you'd like

A recipe using the mask operator. This should live under `Aviation`.

For further details (including about reviewers) please refer to #1550.

### Diagnostic required

Cloud base altitude (ideally should be above ground level), if above sea level orography will be required as well.