Added pressure as a property of an element;
also added a test element with pressure > 1
Added pressure and pressure buffer to world class.
Fixed a small bug early where pressure wasn't allocated.
Changed pressure to a float.
Pressure and decompression code is problematic,
but I don't know why.
Changed default pressure to act the right way.
Changed decompress to not modify changed cells.
Fixed the pressure rules: apparently, I'd been overriding the pressure for air by accident.
Elements2 has become the official set of elements.
Changed high-pressure steam to work a little better.
Goofed the color of high pressure steam. Oops!
Added a number of helpful comments to the elements file.
Fixed an unintentional syntax error.