formula.Formula does not validate maximal charge against total atomic number.

[hanicinecm]

Problem:

Currently, Formula('He+42') happily executes, while the maximal charge should be 2:

In [1]: Formula('He+42').charge
Out[1]: 42

Suggested solution:

Ensure the total atomic number of Formula instance is greater or equal than the charge:

In [47]: formula = Formula('CH4+42')
    ...: total_atomic_number =  sum(formula.atom_stoich[atom.symbol] * atom.Z for atom in formula.atoms)
    ...: if total_atomic_number < formula.charge:
    ...:     raise FormulaParseError(f'Charge {formula.charge} if greater than the total atomic number {total_atomic_number}!')
    ...: 
    ...: 
---------------------------------------------------------------------------
FormulaParseError                         Traceback (most recent call last)
<ipython-input-47-b8fa6fbad41d> in <module>
      2 total_atomic_number =  sum(formula.atom_stoich[str(atom)] * atom.Z for atom in formula.atoms)
      3 if total_atomic_number < formula.charge:
----> 4     raise FormulaParseError(f'Charge {formula.charge} if greater than the total atomic number {total_atomic_number}!')
      5 

FormulaParseError: Charge 42 if greater than the total atomic number 10!

Problems with the suggested solution:

• Some formulas do not define charge
• The mapping between the Atom and Isotope instances from Formula.atoms and their atom_symbols from Formula.atom_stoich.keys() is not defined by atom.symbol, but rather appears to be built up through a series of if ... else cases. However they appear to match.