Reduce default heightmap sampling from 2 to 1

[chapulina]

There's no good reason to subsample by default, see discussion in gazebosim/gz-rendering#180 (comment).

This also needs to be changed in Heightmap.hh once that's merged forward, so I'll keep this PR as a draft until:

• 9 ➡️ 10 #458 is merged
• then a 10 ➡️ 11 PR is merged
• this PR is updated to also change the default sampling on the C++ code

[codecov-io]

Codecov Report

Merging #459 (ebd9ec2) into master (a721bba) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #459   +/-   ##
=======================================
  Coverage   88.10%   88.10%           
=======================================
  Files          66       66           
  Lines        9940     9940           
=======================================
  Hits         8758     8758           
  Misses       1182     1182           

Impacted Files	Coverage Δ
src/Heightmap.cc	100.00% <ø> (ø)

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[chapulina]

This PR is ready for review

[iche033]

changes look good. Just need to update the heightmap test expectations to fix the failing tests

[chapulina]

@scpeters , I'd like to make sure you agree with this change from the physics point of view