Until now the maximum standard deviation in the meshes was used as the
depth. However, the maximum has a strong scatter such that one outling
mesh can completely shift to unreasonable values. So, although the
maximum standrard deviation is still reported by NoiseChisel, but
Makecatalog uses the median standard deviation.

I also added a median function to `statistics.[ch]'.

A tiny spelling mistake was also fixed in MakeCatalog's `--help'
output.