Remove Id markers from source files.

Not used not necessary.
Add docstrings for ScatteringFactorTable classes.

Added docstrings for SFTElectron, SFTNeutron and SFTElectronNumber.
Resolve version from git tag.

Version tagging is now done from git-describe.
Add __gitsha__ variable to the version.py script.
Ignore gdb history file.
Propagate the shared library path on MacOSX.
Allow build customization via sconscript.local.
Replace setXByType with Python property.

Properties for owned registered classes can also accept either
instance or a string that is converted using an instance factory.
Preserving setScatteringFactorTableByType for compatibility with srfit.