Tax convergence check update

Created by: fschreyer

This again modifies the tax convergence check for nash iterations. The criterion is now that tax revenues in all regions must be below 0.01% of GDP in time steps before 2100. Hope this meets the desired precision for regional policy cost etc. I tested SSP2 and SDP runs and it does not significantly increase iterations. If ever this criterion bothers, one can switch it off by setting cm_TaxConvCheck to 0.