Fix discovery of document version number

[heshpdx]

The code at initialization/FGInitialCondition.cpp:1036 is problematic for some newer compilers. I think version was coming back as inf or maybe -inf (I am not sure, it was not on my machine). Reports came in like this:

   In file aircraft/B747/reset00.xml: line 2

   Only initialization file formats 1 and 2 are currently supported
   FATAL ERROR: JSBSim terminated with an unknown exception.

I investigated and tried to figure the intent of the code. It feels like we should only check the version number if it was read in from the file, and otherwise just do a Load_v1. I moved the block around and eliminated the if (version == HUGE_VAL) line which looked fishy (exact FP comparisons are usually not portable).

[codecov]

Codecov Report

Merging #845 (22375ed) into master (ebb770e) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #845   +/-   ##
=======================================
  Coverage   22.43%   22.43%           
=======================================
  Files         167      167           
  Lines       19610    19608    -2     
=======================================
  Hits         4400     4400           
+ Misses      15210    15208    -2     

Impacted Files	Coverage Δ
src/initialization/FGInitialCondition.cpp	40.82% <0.00%> (+0.09%)	⬆️

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[bcoconni]

Overall looks good to me but I think the version variable declaration should now be located in the if (document->HasAttribute("version")) { block (see my review comments).

[bcoconni]

Fine ! Thank you 👍