Fixed major problem with spacegroup finder which would crash for certain cell sizes (usually when they were too small).
Improved performance of inserts significantly.  Previously the entire
database would be searched each time an insert was being performed
to check for duplicates.  Now the filenames of all the structures is
found first and checked against the structures are they are being
inserted.  This way the whole database only needs to be traversed once.
The downside is that if the same structures are assimilated simultaneously
by different processes it is possible that duplicates will make it
into the database.