removed option to remove mesh nodes at nodata values, added default value at nodata values instead
changed dialog so that replacement value can be set from gui
changed static thickness default and do not insert layers with thickness = 0
for multi-layer meshes default values are now set to the node directly above, default value for top layer is set to zero (this might be a problem)
cleaned up the class a bit and fixed a bug
fixes based on PR comments
Merge pull request #278 from rinkk/NoDataReplacement

No data replacement for mesh layer mapping