[CR] Clay and pottery pack. Part 1.

Construction options:
1. Cut Down Grass
2. Remove Dirt Mound
3. Extract Clay Mound
Items:
1. Clay jug
2. Clay bowl
3. Clay container
4. Clay canister
5. Clay pot
6. Clay quern
7. Clay teapot
8. Clay lump
9. Brick
New materials:
1. Fresh Clay
2. Clay
New terrain object:
1. Clay mound
Other changes:
1. Renamed "null" material to "unknown".

(Most of the content took from Extended Cataclysm mod (http://smf.cataclysmdda.com/index.php?topic=5618))
OK, so one tile of shallow water is an infinite source of clay.  I'd kinda like to have it revert to deep water since you dug out the clay, but that's open for discussion.