Removing useless classes in sknano.core.atoms._base module.

Deleting the `AtomList` and `BondList` container classes,
which were useless and only differed by a kwarg to the
class constructor. Also removing _base module imports from
__init__.
replacing AtomList with UserList
'Updating docstrings'
Removing unused code.
updating method docstrings
updating docstrings
Refactoring code to implement Trajectory class.
fix README typo
Fix filter method to use `invert` kwarg.
Simplify `filter` method code and update docstrings.
Adding new method `mapatomattr`.

This method provides a means of updating an atom attribute
such as the atom element or setting a new atom attribute
using an existing atom attribute and a dictionary mapping
the existing attribute to the value of the updated or new
attribute.
Updating docstrings.
Adding new modules for POAVAtom/POAVAtoms classes.

Adding POAVAtom and POAVAtoms classes for POAV structure analysis.
Adding test modules for testing POAVAtom and POAVAtoms classes.
Adding module to hold references to StructureAtom(s) classes.

Adds `StructureAtom` and `StructureAtoms` to package namespace
which point to the Atom and Atoms sub-classes that should
be imported and used by the sknano.generators and sknano.io
package modules.
Updating module imports for StructureAtom/StructureAtoms classes.
Adding imports from _poav_atom and _poav_atoms modules.
Removing POAV analysis attributes from KDTAtom/KDTAtoms classes.
Updating package docstrings
updating package docstrings