Documentation updates.

[bcoconni]

This PR addresses 2 issues related to documentation:

• Issue #922 where the build process crashes if the Python documentation generation is triggered.
• Issue #1037 where the documentation is misleading in claiming that mixture position and command shall be constrained within the range from 0.0 to 1.0

Regarding the first issue, the problem is due to the lack of Doxygen documentation in src/simgear/props/props.hxx which makes python/Doxy2PyDocStrings.py crash. The problem has been fixed by including the documentation of FGPropertyNode.get_double_value() and FGPropertyNode.get_double_value() directly in python/JSBSim.pyx.in.

In the process, FGPropertyNode.set_double_value() has been fixed to behave like its C++ counterpart (returns a boolean to indicate if the update of the property value has succeeded).

[davidmegginson]

Since all of the other control inputs are normalised between 0.0 and 1.0 (or -1.0 and 1.0), it seems strange that the mixture wouldn't be, but understood why that's necessary to work around an issue in one of the FDMs.