add SystemProperty table to database

the new table is for storing system-wide properties like the number of atoms, potential parameters, or the number of frozen atoms.  The data can be stored in an easy to read manor as int, float, string, or a picked object that will need to be imported into python to be read.

Also take the opportunity to remove the Distance table.  

Niether of these changes require the database to be "migrated".  sqlalchemy will add the new table SystemParameters automatically, and ignore the (now deleted) table Distance, if it already exists.