Add wetting and drying

[jpolton]

Wetting and drying permits the water line to move. It also relaxes the requirement for a "minimum depth" being hardwired into the bathymetry (typically about 10m)

[jpolton]

Need to:

• Flick wetting and drying switches to turn it on
• Test run
• Regenerate bathymetry file to relax 10m minimum depth requirement
• Test run