Renaming lammps_atom_styles dict to atom_styles.

Adding lammps_atom_styles variable reference to atom_styles
dict for backwards compatibility.
Removing 'nx', 'ny', 'nz' list elements from attribute lists.

Instead of hardcoding these 3 list elements into the atom_styles
attribute lists, the existing lists are extended with a list
of these 3 elements in a loop after the starting lists are
defined.
Renaming `lmps_atom` variable to `line` for simplicity.
updating tests for lammps_data_format
Refactoring headers dictionary to remove redundant code.
Refactoring variable definitions.

Changing header_spec and section_header_map dictionaries to
global module variables instead of class variables defined
in DATAFormatSpec class.
updating lammps_data_format tests
Moving UserList container class to separate module.

Moved the `UserList` class from sknano.core.atoms._base
module to its own module sknano.core._user_list as it is
of more general use to packages outside of sknano.core.atoms
sub-package.
Adding new `molecules` sub-package in sknano.core
Updating `UserList` import
Updating docstring directives and package level imports.
Adding new modules to molecules sub-package.

Adding sknano.core.molecules._molecule and
sknano.core.molecules._molecules from the sknano.core.atoms
atom and atoms modules as starting templates for future dev.
adding test modules for sknano.core.molecules package
Adding module imports for molecules subpackage.
Adding new module for atom trajectory class development.
updating atoms sub-package module imports
Adding new modules for developing new MDAtom/MDAtoms classes.

MDAtom/MDAtoms will be derived from the KDTAtom/KDTAtoms
classes and will be developed for analyzing the trajectories
of structure data atoms from molecular dynamics simulations.
adding boilerplate code for Trajectory class constructor
adding test module for Trajectory class