Move the Mach/VCAS conversion methods to FGAtmosphere

[bcoconni]

Following the on-going process of removing static variables and methods as motivated by the issue #666, this PR moves the conversion methods between the Mach number and the calibrated airspeed (VCAS) to the FGAtmosphere class. This allows StdDaySLsoundspeed no longer being a static member and removes 4 static methods from FGJSBBase.

In addition the conversion methods VcalibratedFromMach and MachFromVcalibrated now use the altitude rather than the ambient pressure as an input parameter. This change makes the methods consistent with other methods from FGAtmosphere. In particular this avoids interrogating the FGAtmosphere class for the ambient pressure before calling a static method from FGJSBBase which simplifies the code in FGInitialConditions.

"Magic" constants have also been replaced by their expression using the specific heat ratio $\gamma$ as found in classical textbooks. Most of these computations use constexpr meaning that the constant is computed at compile time. These changes have therefore no performance impact in most cases (as the compiled code use a constant) but this improve the code legibility and allows to document how the constants are computed.

Finally the unit tests for these conversion methods have been rewritten and now check that the computation made by the conversion methods are consistent with thermodynamics:

• Mass conservation $\rho_1u_1=\rho_2u_2$
• Momentum conservation $p_1+\rho_1u_1^2=p_2+\rho_2u_2^2$
• Energy conservation $C_pt_1+\frac{1}{2}u_1^2=C_pt_2+\frac{1}{2}u_2^2$

where $p$ and $t$ are static pressure and static temperature. This allows removing the usage of hardcoded values and therefore testing a greater range of values and scenarios.

[codecov]

Codecov Report

Merging #881 (6368b22) into master (4f3c0e6) will decrease coverage by 0.18%.
The diff coverage is 87.80%.

@@            Coverage Diff             @@
##           master     #881      +/-   ##
==========================================
- Coverage   23.24%   23.06%   -0.18%     
==========================================
  Files         167      167              
  Lines       19607    19611       +4     
==========================================
- Hits         4557     4524      -33     
- Misses      15050    15087      +37     

Impacted Files	Coverage Δ
src/FGJSBBase.cpp	72.22% <ø> (-15.28%)	⬇️
src/FGJSBBase.h	90.32% <ø> (ø)
src/models/FGAtmosphere.h	100.00% <ø> (ø)
src/models/FGAuxiliary.cpp	61.18% <0.00%> (-0.92%)	⬇️
src/models/FGAuxiliary.h	84.90% <ø> (ø)
src/initialization/FGInitialCondition.cpp	44.85% <50.00%> (-0.50%)	⬇️
src/FGFDMExec.cpp	41.20% <100.00%> (-0.20%)	⬇️
src/models/FGAtmosphere.cpp	94.58% <100.00%> (+0.97%)	⬆️

... and 16 files with indirect coverage changes

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[seanmcleod]

Glancing at the diffs it looks good.

[bcoconni]

Glancing at the diffs it looks good.

Thanks ! The PR is merged.