Add unicode form formula

[montmorill]

eg.:

from chempy import balance_stoichiometry, Reaction
from chempy.printing import unicode_

reac = {"H+", "SO4-2"}
prod = {"H2SO4"}
reac, prod = balance_stoichiometry(reac, prod)
print(unicode_(Reaction(reac, prod)))

before: 2 H+ + SO4-2 → H2SO4
after: 2 H⁺ + SO₄²⁻ → H₂SO₄

[bjodah]

For simple substances this is a nice change of default. However, I do worry that for other substances which are typically entered by their field specific code name (which may contain digits), e.g G6P for glucose-6-phosphate, then this default might be unwanted. We have a .unicode_name field on the Substance class. But it is only set by the parser I think.

Perhaps something along these lines would be preferable (I'm thinking out loud here, I'm not asking you the implement the following):

>>> r1 = Reaction.parse("H+ + SO4-2 -> H2SO4", elements_in_formula=True)
>>> r2 = r1.balanced()
>>> r2
2 H⁺ + SO₄²⁻ → H₂SO₄

That way, someone who is entering e.g. "G6P" may pass a different flag to the .parse method. What do you think?

I find that printing options in general are a bit tricky, since the preferred defaults may differ quite a bit between different fields...