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.