Introduce 'do not create attribute table' flag to r.contour

[wenzeslaus]

G7:r.contour always creates attribute table, although the vectors are 3D vectors and the height/value is stored as z coordinate.

The following modules have (consistently) -t flag:

• G7:v.to.points
• G7:v.overlay
• G7:r.to.vect
• G7:v.in.lidar
• G7:v.in.dxf
• G7:v.voronoi
• G7:v.in.ogr
• G7:v.select

-t
    Do not create attribute table

It is bit strange that -t means no attribute table but at least it is consistent (at least among these modules).

This can improve #1626 if it is still an issue and it will be much faster then creating attribute table (even when r58804 (write attributes in one commit) is considered).

[annakrat]

Flag -t implemented in r60381. Just to make sure I will backport it after a few days.

[annakrat]

Thanks for r60386. Backported in r60488.