Grate shaking update

Grate.pde - Modified and refactored grate shaking logic.  Now uses a
timer to time shaking, instead of grate_val.  Removed
grate_pratio_accumulator, since it didn't do anything.
Reactor.pde - Moved p_ratio assessment calculations here, since this
shouldn't be the responsibility of the grate shaking code.
KS_PowerPallet.pde - cleaned up comments, added intmath.h include,
modified grate shake constants for new grate code, move grate shake
variable initialization to Grate.pde.
Display.pde - slightly modified ash auger display.  Moved grate mode
logging to Grate.pde.  Remove automatic return to GRATE_SHAKE_PRATIO
mode when exiting the manual control menu and shaker is off.