[PR] Gravity and Gravity Tethers

[glowbold]

Rebalance Focused on High-G:
* Internal: Convert all floats to ints all my homies hate floats
* Balance: Being submerged in fluid neutralizes per-tick impacts of high gravity
* Balance: Being submerged in fluid gives you full traction
* Balance: Jetpacks prevent low-G stamina sapping for humans
* Balance: Super-extreme-G interferes with human sight.
* Balance: High cap on per-tick damage (previously uncapped)
* Balance: Fitness machines scale stamina buffs based on Gs directly instead of a differing bonus at different gravity thresholds
* Balance: Floating atoms resist high-G
* Balance: The "Space Movement" equipment property has been converted to "Zero-G Movement" and works in all zero-G environments

[Mikeistaken]

I think the station wide tether should have the amount of materials needed reduced

Reason for request:
Miners generally cant get Quill consistently and most people don't know that Erebite works as extraordinarily dense crystal, lowering this number will allow for another gravity tether to be made, especially when the miners get their full equipment and decide to go to some random place

[glowbold]

Testmerge updated - these should be available in the next round or two from this post time depending on when rounds end.

Balance updates:
A few ways of being moved didn't give inertia, which left people stuck in zero-G more often than intended. These changes should make gravity feel more a little more "fair" and realistic.
* Pushing someone away in Zero-G gives you both inertia in opposite directions
* Swapping positions with someone gives them inertia in the opposite direction

Tether costs made station tethers difficult to build and prevented casual or antag use with local tethers. These changes are intended to make it easier to build/rebuild gravity tethers in early/mid-round with a lesser reliance on rare mining ores.
* Station tether cost change 
    * FROM 5 metal, 5 dense crystal, 4 super-dense metal, 3 koshmarite, 2 energy extreme
    * TO 5 metal, 1 dense crystal, 3 super-dense metal, 3 koshmarite, 4 energy high
* Local tether cost change 
    * FROM 2 metal, 1 dense crystal, 1 super-dense metal, 1 koshmarite
    * TO 2 metal, 1 super-dense metal, 1.5 koshmarite, 1 energy high

Bug Fixes:
* Conveyors should feel less weird to run against their direction of travel