Mark a separation legs for breaking up the tunnocks/balcon into a third layer.

[goatchurchprime]

This is to support the methods outlined here: https://github.com/CaveSurveyGIS/CUCCexposurveyissues/issues/43#issue-393537276

The implementation in the svx data might be in caves-1623/204/subsoil/whereowhere.svx

14  15  7.73  303  -32

*title "000 separation leg"
16  15  5.43  107  +20

*title "Where O'Ware"
16  17  10.05  264  -30
18  17  11.78  162  -27

Should also add a *title to subset command, like

*title "000 component leg: tunnocksupper" to mark out each connected component of the system once the separation legs have been deleted.

[goatchurchprime]

done