Add formula.element to accumulate across isotopes and ions

[pkienzle]

The existing formula.atoms attribute keeps the isotopes + charges separate when accumulating the components.

This proposal uses formula.elements to accumulate across isotopes and charges. It also adds formula.elements_hill to return a structure in hill notation order.

>>> print(formula("HDS{6+}O{2-}3O[16]{2-}").elements_hill)
H2O4S

Having written this, several questions arise:

1. Will there be confusion between formula.elements and formula.atoms?
2. Do we want formula.isotopes for accumulating isotopes without charge?
3. If we are accumulating isotopes, do we want to turn natural abundance into isotope distributions?

I made the underlying count_elements() function public and gave it a by_isotopes flag.

>>> from periodictable.formulas import count_elements, formula
>>> print(formula(count_elements("HDS{6+}O{2-}3O[16]{2-}")))
H2O4S
>>> print(formula(count_elements("HDS{6+}O{2-}3O[16]{2-}", by_isotope=True)))
HDO3O[16]S

I'll await comments from those who can use this functionality before merging.

Refs #14

[pkienzle]

Backing off on the elements/elements_hill attributes, and only providing the count_elements function. This keeps the interface slightly smaller and maybe less confusing than having both .elements and .atoms.