combine DXF and GeoEasy data sets to create DTM

[zsiki]

The radio buttons in the DTM creation dialog should be changed to check button. The input from the different sources should be merged, removing repeated points.
This issue was proposed by Jofe Graham-Jenkins.

[zsiki]

There is a solution to get the combined DTM from DXF file and coordinate list in three steps.

1. Create a DTM from the DXF file and close it.
2. Load the coordinate list, add break lines, boundary and create another DTM
3. Select Add from the DTM menu and select the DTM from DXF file

The second DTM will be extended with the points, break/border lines of the first DTM. This process works not only to combine DXF and coordinate list. You can combine any two sources (e.g. two DXF files).