Correct units for pressures to reflect normalisation used in the code.

[mrhardman]

Units for the output binary file appear to be incorrect for the pressure. Here I have corrected the units to what I understand the normalisations are.

The old normalisation for p was

$$p_{ref} = n_{ref} T_{ref}$$

the corrected normalisation is

$$p_{ref} = n_{ref} m_{ref} c_{ref}^2$$

with $c_{ref} = \sqrt{2 T_{ref}/m_{ref}}$.

@johnomotani We should also check that the normalisation reported for the heat flux is correctly documented, since the units there also reference $T_{ref}$ and not $m_{ref}c_{ref}^2$.

May close #136.

[johnomotani]

I think this is correct. I've applied a similar update to the units for heat fluxes.