Fix wind initial conditions

[bcoconni]

This PR fixes a 12+ years old bug where the input value was not converted from kts to ft/s.

void FGInitialCondition::SetWindDownKtsIC(double wD)
{
  const FGMatrix33& Tb2l = orientation.GetTInv();
  FGColumnVector3 _vt_NED = Tb2l * Tw2b * FGColumnVector3(vt, 0., 0.);

-  _vt_NED(eW) = vUVW_NED(eW) + wD;
+  _vt_NED(eW) = vUVW_NED(eW) + wD * ktstofps;

The PR contains an update to the unit test FGInitialConditionsTest.h to prevent regressions. I also took this opportunity to fix a couple of details:

• The constant ktstofps is computed to the floating point precision of double (thanks to the C++ constexpr feature, the computation is done at compile time, i.e. once for all)
• The private method GetNEDWindFpsIC has been removed as it was redundant with GetWindNEDFpsIC.

[codecov]

Codecov Report

Merging #855 (6cd73ed) into master (b2172a4) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #855      +/-   ##
==========================================
+ Coverage   22.43%   22.60%   +0.17%     
==========================================
  Files         167      167              
  Lines       19608    19602       -6     
==========================================
+ Hits         4400     4432      +32     
+ Misses      15208    15170      -38     

Impacted Files	Coverage Δ
src/FGJSBBase.h	90.32% <ø> (ø)
src/initialization/FGInitialCondition.cpp	45.12% <100.00%> (+4.29%)	⬆️
src/initialization/FGInitialCondition.h	74.71% <100.00%> (-1.38%)	⬇️
src/input_output/FGXMLElement.cpp	47.33% <100.00%> (ø)

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