Remove the flag BASE from string_utilities.h

[bcoconni]

For some reasons the file string_utilities.h was meant to be used both as a header and as a c++ source file. The flag BASE had to be defined when the source code of the functions was to be compiled. Otherwise, when BASE was not defined, only the function signatures were used as if the file was a "pure" header file. This was like so since the file string_utilities.h had been included in JSBSim in June 2009.

This PR is meant to remove this unusual process: string_utilities is split into a pair of .h and .cpp files just like the rest of the code base.

• Only function signatures are kept in string_utilities.h
• The C++ code is moved in string_utilities.cpp
• The flag BASE is removed from the code base.
• All the functions from string_utilities.h are prepended by JSBSIM_API to allow calling them in unit tests while JSBSim is compiled as a DLL.
• Because all the functions are now exported, they have been moved to the JSBSim namespace to avoid name collisions with other libraries.

[codecov]

Codecov Report

Merging #837 (c875e39) into master (08a3ae5) will not change coverage.
The diff coverage is 98.11%.

@@           Coverage Diff           @@
##           master     #837   +/-   ##
=======================================
  Coverage   22.37%   22.37%           
=======================================
  Files         168      167    -1     
  Lines       19535    19535           
=======================================
  Hits         4371     4371           
  Misses      15164    15164           

Impacted Files	Coverage Δ
src/FGJSBBase.cpp	87.50% <ø> (ø)
src/input_output/FGXMLElement.cpp	47.33% <ø> (ø)
src/input_output/FGfdmSocket.cpp	0.00% <ø> (ø)
src/input_output/string_utilities.cpp	97.36% <98.11%> (+1.71%)	⬆️

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