Support Unicode characters for identifiers (parameter names) in formulas

[msrocka]

The formula interpreter currently throws an exception when other letters than [a-zA-Z] are used:

We have to modify the parser grammar for this: https://github.com/GreenDelta/olca-modules/blob/master/olca-formula/src/main/java/org/openlca/expressions/FormulaParser.jj#L94