Add images for ST_ZMax and ST_ZMin
Update images for ST_ZMax and ST_ZMin

Conflicts:
	docs/dev/ST_ZMax.png
	docs/dev/ST_ZMax.svg
	docs/dev/ST_ZMin.png
	docs/dev/ST_ZMin.svg
Merge pull request #95 from gpetit/st-x-min-max

Add images for ST_ZMax and ST_ZMin