GoCAD interface fixes
Three small fixes for the GOCAD ASCII interface
- surface names allow chars '/' and '' within the name which prevents the writing of files (at least on Windows), so these are now replaced with '-'
- the REGION-tag is now recognised (and ignored), so the interface doesn't throw a warning when finding it
- the ZPOSITIVE-tag is now correctly interpreted (i.e. if set to DEPTH, z-coordinates are now inverted)