Correct VEAS to use standard day SL density

[seanmcleod]

Slightly messy layout in FGAtmosphere.h in terms of ideally wanting to add StdDaySLdensity to the existing group of public constants.

  static constexpr double StdDaySLtemperature = 518.67;
  static constexpr double StdDaySLpressure = 2116.228;
  const double StdDaySLsoundspeed;
  static constexpr double SHRatio = 1.4;

But require Reng0 to be defined first, but it's currently lower down in a protected section.

So couple of options in terms, not sure if the one I've chosen is the best.

[codecov]

Codecov Report

Merging #900 (31ca720) into master (02b4e25) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #900      +/-   ##
==========================================
- Coverage   23.10%   23.09%   -0.01%     
==========================================
  Files         167      167              
  Lines       19627    19626       -1     
==========================================
- Hits         4534     4533       -1     
  Misses      15093    15093              

Impacted Files	Coverage Δ
src/FGFDMExec.cpp	41.40% <ø> (-0.09%)	⬇️
src/models/FGAtmosphere.h	100.00% <ø> (ø)
src/models/FGAuxiliary.cpp	66.00% <0.00%> (ø)

[bcoconni]

Looks good to me.

[bcoconni]

But require Reng0 to be defined first, but it's currently lower down in a protected section.

Not sure there is much reason for this constant to be hidden behind a protected wall. At some point maybe it should be renamed to R_dry_air or any name that is more explicit than Reng0 ?

So couple of options in terms, not sure if the one I've chosen is the best.

You've chosen the most conservative which is fine as far as I am concerned 😄.