Gravity Generator

- Addition of Gravity Field Generators
- Added animation for when floating
- Toggle Gravity in Secrets menu only works when a Grav Gen is on thestation
- New debug verb to reset the gravity on the whole server

Because of the way this has been implemented, any Grav Gen on any zLevel
will affect the station level only. I would need to rewrite the whole movement/gravity system to make it per level. Something I think can be saved for if/when we switch from station to outpost.

If for any reason the server looses gravity, there is a debug verb to reinstate the default gravity mechanic.

Mapping info:
At the start of round there doesn't need to be one on the station as the
original gravity system will be running when the round starts.  As soon
as a Grav Gen has been spawned and it has started up (Done automatically
if already on map or spawned using the /station/admin version) It will
take over all gravity simulation.

The main object:
/obj/machinery/gravity_field_generator/main

The one to be place on a map at round start:
/obj/machinery/gravity_field_generator/main/station

The one admins should spawn if they need/want to spawn one:
/obj/machinery/gravity_field_generator/main/station/admin