Adding 'dump' extension to list of supported structure formats.

Also adding DUMPReader/DUMPWriter checks to
StructureReader/StructureWriter class methods.
Adding the self.fname assignment back into code.
Adding test fixture class to tools module in testing package.
Updating sknano.testing package docstrings and module imports'
Update sknano.generators tests.

Implementing GeneratorTestFixtures class into sknano.generators
test modules, which now automatically removes the generated
structure data files.
Adding fx,fy,fz atom attribute data types to attr_dtypes dict.
New code and development on LAMMPS dump format reader/writer
updating test module for lammps dump format testing