Improve output defaults (list of nits)

[gdt]

I find that several things could be smoother:

1. With input layer "topo" with height Z($geometry), output is topo_expr. A nicer default would be "topo-contours-1m", for 1m interval.
2. With 1m interval, label precision defaults to 3. I suggest the smallest precision s.t. round(interval, 6) == round(interval,p), or at most 3, which would be 0 for 1m, and 1, for 0.1m or 0.2m.
3. After applying suggestions 1/2, I get a scratch layer "topo-contours-1m". When I click the bug icon to save it, the suggested name is "topocontours1m", losing the hyphens. But hyphens are fine in geopackage layer names.

(I don't mean to gripe - these are minor suggestions offered in the hope they are helpful, from the perspective of someone trying this the first time. Thank you for maintaining this, and the big point is that by omission, everything else worked great.)