Do not allow nodes to be created within walls

[goatchurchprime]

Apparently it is not recommended to put nodes within walls as it can make merging two sections of survey together a bit noisier and also if you do it then forget to remove the invisible lines at the end of the surveys when you join them this can also lead to problems.

A simple way in future to stop people putting nodes in walls would be to only allow connective lines to come from survey station nodes.

Duncan's suggestion.