Add Oxygen Supply functionality to US cores. (For GH #9)
Updated US Fuel Cell (#9)

- Switched to a TAC generator
- Added missing water tank for fuel cell
- Added description to fuel cell with power rating and water output.
Swapped LOX and LH2 tank sizes in the US cell.

For GH #9.
Added descriptions for US tanks and Fuel Cell (#9)
Added LS description to US Radial Oxygen (#9)
Updated Elektron (US, GH #9)

- Updated to TAC generators.
- Updated to match discussion on GH #9. Previously it was only
  using 4litres of water per day.
Merge pull request #11 from pjf/US

Updates to Universal Storage (#9)