CO2_max_total should use years_represented

[jkiviluo]

CO2_max_total is supposed to be a limit on cumulative emissions (i.e. carbon budget). However, it does not have any scaling and needs to be therefore manually adjusted to the amount of time that is being modelled (investment runs typically use representative periods and to size the limit, it needs to take modelled time into account).

Most of the time, one should just use CO2_max_period, since this is automatically scaled to give an annual constraint (even when modelling couple of weeks).

But to make CO2_max_total more intuitive, it should be scaled to represent the years_represented (sum of the year_represented in all periods that are modelled). Also comment on this in the parameter description (and make a how-to).