* Make it mandatory to provide a period when calculating a variable.
- When a computation is requested (with *.calculate_output, entities.__call__, *.calculate[_add|_divide], *.compute[_add|_divide]), the argument `period` is no longer optional.
- Previously, the period of the simulation was used instead of a missing period. It was error-prone, as values would be returned for the wrong period, without any error or warning to alert the formula writer about a likely coding error.