Large internal lakes: Add support for multiple regions & sources of contour data

[rosepearson]

With the current setup you can include offshore contours or lake contours but not both at the same time. Provide support for this. Will need to give some thought as to if there is a different 'lake' option that means that the lake doesn't need to be cut out of the land outline.

Two example lakes are:

• Taupo
• Wairewa