private Value evaluateSingleFormula(final Formula f, final Function<String, Value> varMap) {
        try {
            return f.evaluate(varMap);
        } catch (FormulaException fe) {
            return null;
        }
    }
Evaluates both the double value and the CharSequence representation of this DegreeFormula
to allow for a more performant calculation.

@param varMap varmap to calculate values for
@return pair with calculated double value as first param (might be null) and text representation as secod (never null)
main/src/main/java/cgeo/geocaching/utils/formulas/DegreeFormula.java