Fix vapor enthalpy calculation in simpleh2o.hh

What happened / Problem description:

I encountered a discrepancy in the calculated gas enthalpy and liquid enthalpy of water between the simpleh2o.hh and h2o.hh files. Upon reviewing the code with Mathis, we identified a potential error in the vaporization enthalpy calculation within the simpleh2o.hh file. Specifically, the calculated vaporization enthalpy seems to have been overlooked for conversion from kJ/kg to J/kg, as it requires multiplication by 1000.

What you expected to happen:

The gas and liquid enthalpy values calculated using simpleh2o.hh should align with those obtained from h2o.hh, provided that the reference temperature is the same.**

How to reproduce it (as minimally and precisely as possible):

check /dumux/material/components/simpleh2o.hh at line 145