Fix: Don't polute global namespace with std.

[cbirkhold]

using namespace ...; in a (especially a public) header file is bad form.

props.hxx does collapse the std namespace into the global namespace which can lead to problems in client libraries.

Of course there may also be client code out there now that depends on this which would require that code to either add std:: in some places or add using namespace std in the right places (which hopefully is inside another namespace or a compilation unit.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 24.96%. Comparing base (9f64ced) to head (70eb717).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/input_output/FGPropertyManager.h	33.33%	2 Missing ⚠️
src/models/propulsion/FGTurbine.h	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1193   +/-   ##
=======================================
  Coverage   24.96%   24.96%           
=======================================
  Files         170      170           
  Lines       19283    19283           
=======================================
  Hits         4814     4814           
  Misses      14469    14469           

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[bcoconni]

Good catch ! PR merged.