improves grass7 v.in.dxf making a couple of parameters optional/mandatory

Author: gioman

python/plugins/processing/algs/grass7/description/v.in.dxf.txt

@@ -1,8 +1,8 @@
 v.in.dxf
 Converts files in DXF format to GRASS vector map format.
 Vector (v.*)
-ParameterFile|input|Name of input DXF file|False
-ParameterString|layers|List of layers to import|
+ParameterFile|input|Name of input DXF file|False|False
+ParameterString|layers|List of layers to import||False|True
 ParameterBoolean|-e|Ignore the map extent of DXF file|True
 ParameterBoolean|-t|Do not create attribute tables|False
 ParameterBoolean|-f|Import polyface meshes as 3D wire frame|True