remove mechanic job and merge with engineering

[Erev]

I'd prefer keeping the engineer and mechanic roles separate but engineering DOES need more options for after they've got the engine up and running.

I also personally believe that Science should be split up between geneticists (under MD already which is fine), physical scientists which focus on chem and toxins (also give toxins non-traitor usefulness please), and xenoarchaeologists which focus on artifacts and telescience.