This commit enforces the following naming conventions:

    element = element_symbol(argument)
    isotope = isotope_symbol(argument)

Using `symbol` leaves it ambiguous whether it refers to an element,
isotope, or ion.

The new atomic_symbols dictionary in elements.py has atomic numbers as
keys and the atomic symbols as items.  This results in the same
functionality, but will not break if we remove neutrons from index 0.

New functions include: __is_hydrogen, __is_positron, __is_electron,
__is_antiproton, __is_neutron, and __is_alpha.  These functions check
whether or not a particle string corresponds to any of the cases with
special names.  These functions are designed to make functions like
atomic_symbol more readable.  This is still a work in progress.

This fixes a bug where __extract_charge_state('H-1-') did not return
('H-1', -1).

Up until now, atomic symbols were case-insensitive (e.g., 'd' and 'D'
both stand for deuterium; but 'p' means proton and 'P' means
phosphorus).  The problem with case-insensitivity is that it allows
for sloppier and less consistent code.

Because: "coverage/coveralls — Coverage decreased (-0.01%) to 98.955%"