_getCheckTypeHelper(functionName, valueType) {
        return value => {
            if (typeof value !== valueType) {
                throw new TypeError(`Sheet.${functionName}: invalid type - value must be of type ${valueType}.`);
            }
        };
    }
Get a helper function to check that the value is of the expected type.
@param {string} functionName - Name of the parent function.
@param {string} valueType - A string produced by typeof.
@returns {function} The helper function, which takes a value. If the value type is not expected, a TypeError is thrown.
@ignore