Hydroponics fixes, #2992, #3002, chemicals, attackby()

Tray's chemical procession is made into a separate proc. Attackby is a bit less horrible.
Nutriments again affect plant's mutations and yield. They are now actual chemicals. Slightly poisonous, too.
Containers will no longer splash their contents at trays and biogenerators (fixes #2992 and #3022).
Almost all spawn() removed from grown creation. Yay.
Irrigation won't start with pills (and syringes, and growns), or when tray is not connected.