Added support for setting prices on planets.

I notice that one of BTAxis' complaints about this economy system
was that you can only set prices at the system level. I kind of
agree that it's inconvenient, since some planets might have special
properties, but I also think setting prices at the system level has
some value, so rather than moving price adjustment to the level of
planets entirely, I simply added an extra ability to set prices at
the level of planets. If you do this, the system-wide settings are
overridden.

Planet-level prices don't spill over to neighbors the way
system-level prices do.

Also added Lua functions corresponding to planet-level price
control, and renamed the existing system-level ones to make what
they do more clear. While I was there, I cleaned up the Lua
functions and especially the Lua functions' documentation, which
was sloppy and often incorrect.