bumping version to v0.3.7.dev for development
fixing travis-ci install config
tweaking travis-ci config pip install commands
adding before_install commands from scipy travis.yml config
Changing mass attribute variable name from 'm' to 'mass'.
adding pe, ke, etotal attributes to XAtom/XAtoms classes
removing duplicate mass property methods from _extended_atom module
adding new method `get_atomtypes`
removing old commented out code
Updating XAtom class __str__ and __repr__ methods.
updating _extended_atoms docstring
Updating _lammps_dump_format DUMPReader class methods.

Removing all non-functional methods and refactoring unscale
method.
removing commented out line of code
changing default value of `nz` to `None`
adding sigma_bond_angles to `todict` method dict
removing Python 3.3 support
Removing call to `config.make_config_py`.

Calling this function installs a __config__.py module which
breaks some setup commands.
Removing the git revision string from the full version string.
updating docstrings
Updating call signature of compute functions.

Replacing positional arguments `n`, `m` with varargs `Ch`
which can be a tuple or 2 ints.