Fixed precision in thermal expansion coefficient calc.

Loss of accuracy was caused by different sampling
of volume and enthalpy and as a result alpha was
computed incorrectly. With the present "fix" the volume
and enthalpy are both downsampled to what is written
in the .edr file. The real fix would be to store the product
of H and V in the .edr file, but that falls outside the
4.6 branch policy.

Change-Id: I1be06d689002d7c9d6be92bf1e377912f0be1efd