Commented out Hydra-spawning

Hydra's no longer spawn in the monster branch. Any further work on them
will be done in boss.

Giant Amoeba's hasn't been commented out due to them being a regular
monster. The (not yet implemented) Colossal Amoeba boss will rely on the
code for the Giant Amoeba's split-on-death ability though.
fix on-death for giant-amoeba

add-* functions return a map of the system and z, so you have to extract
the system when you're passing it further along.
add a message for giant amoeba splitting
spawn multiple large amoebas on giant amoeba death
move giant amoeba split rate to config.clj
can now change spawn rate just from giant_amoeba-split-rate in config.clj
add colossal amoeba
Merge with Monster