Bug: r3.mapcalc crashes with non-cubic volumes.

Reason is probably a bug in z() which obviously reads
ew or ns instead of top-bottom.

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This r3.mapcalc implementation is partly outdated (ignoring
several GRASS lib functions).

For upgrade current r.mapcalc should be used and the 3D
extensions merged into. That might be less work than
fixing all the oldish code here.

2/2002 MN
