Added possibility to run with an Helium "he" tracer (to be initialized
at constant value of 3.6e-7 kg/kg_air, i.e. the 4ppm of Krasnopolsky 1996
EUVE satellite, using newstart).
corrected case for CH4 in aeronomars/photochemistry.F (was using undefined
indexes when there is no CH4).
updated aki/cpi coefficients for Argon used to compute mean atmospheric
Cp and thermal conductivity in aeronomars/concentrations.F