Apply CCFLAGS filter to string flags only.

Latest scons may have tuples in CCFLAGS.  good_python_flags filter
now let's them pass.
Ignore ctags indices.
Fix link flags for python c++ extension on Darwin.
ignore compiler error file.
Add helper class for safe registration of Python-extended C++ classes.

This class can be used as a configurator for classes that derive from
HasClassRegistry and provide the _registerThisType method in Python.
This needs to increase reference count for the prototype object in the
registry, so it is not destroyed when Python exits before the shared
library with the registry.
Rename class to wrapper_registry_configurator

This is a helper class not to be used anywhere else.
Return SharedPtr from the fetch function.

wrapper_registry_configurator.fetch() now return the converted
smart pointer to the wrapped C++ class.  No need to convert
Python object twice.
Fix C++ registry storage extended classes.

_registerThisType() method now increases the reference count for the
Python class instance that is in the C++ registry.  This prevents crash
when Python quits before deallocating the C++ registry in a shared
library.
Fix typo in the assert statement.
Register CovalentRadiiTable on module import.

Also delay the import of periodictable until the first call
of CovalentRadiiTable.lookup.  This allows to register this class
even if periodictable is not installed.
Remove obsolete unused functions.
Wrap the concrete ScatteringFactorTable classes.

Added Python-wrapped classes for SFTXray, SFTElectron,
SFTNeutron and SFTElectronNumber.  All with pickling support.
Add docstring for the SFTXray class.
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.