phase.gp: Add gnuplot script and rules to build a phase-space plot

phase.gp: Add gnuplot script and rules to build a phase-space plot

In phase.gp, I use a for loop to iterate over data block (in this
case, each trajectory is in its own block).  This gives you nice
per-block colors, but means you'll have to update the loop if you
change the number of energies you're looping over.