[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))
Probably a good idea generally, since all the clay vessels have the SEALS flag, and IME clay lids are never watertight for transit.