Add a parameter for selecting for calculating the charge methodulation

[sdmytrievs]

Global default settings for calculating the charge method.
If get_charge_from_formula is false, calculate the charge based on the elements and their default
or specified valence; otherwise, take the charge based on the symbol in the given formula.

[sdmytrievs]

1. Added for C++ and Python API the properties function and dependent on it functions parameter for selection of type charge calculation:

/// Calculate charge, molar mass, elemental entropy, atoms per formula unit
 /// for chemical formulae.
 /// If use_formula_charge false, calculate the charge based on the elements and their default
 /// or specified valence; otherwise, take the charge based on the symbol in the given formula.
 FormulaProperties properties(const ElementsData& dbelements,
                                 bool use_formula_charge = charge_from_formula());

2. Added global settings for the default type of charge calculation and set/get functions for changing it.

bool charge_from_formula();
void set_charge_from_formula(bool cond);

3. Implemented tests for C++ and Python API

[gdmiron]

Is it possible to return false or some error/777777 if formula charge and charge from formula do not agree?

Please add example of formula charge to main readme.

[sdmytrievs]

1. Into the function properties, implemented an info message if a charge imbalance:

[chemicalfun] [info] In the formula: Am|3|(Oxa)3-3 calculated charge: 3.000000 != -3.000000
[chemicalfun] [info] In the formula: CaC2 calculated charge: 10.000000 != 0.0000000

2. Changed implementation of the function testChargeImbalance. Now, it can return true if a charge imbalance is detected or throw an exception, depending on the second parameter.

[gdmiron]

I have checked the usage with:

as you can see the token.charge() always returns the charge calculated base on valence.
Maybe we need this use_charge_from_formula at a level of setting the formula token. By default this is False.