Added carbon diamond test.

Note that Ecut = 128, which is very, very large.  This is to make
sure the pressure is really converged (which it still isn't quite).
Tests for higher/lower ecut.

I thought I should back up claims that 128 Ry is nessesary for
'full' convergence.  The standard I'm using here is numerical,
not physical: the stresses are identical to 3 printed decimals.
At Ecut = 64, the difference in stress is 0.08, which is
more than good enough for physical agreement.