Tax conv

Created by: fschreyer

This would introduce a convergence check for tax revenues in the Nash algorithm. In addition to the prior criteria, Nash only stops now if abs(vm_taxrev) < 1e-3 for all regions and time steps. This 1.) helps detecting problems when you implement taxes as tax convergence was not automatically checked before. 2.) It lets all tax revenues converge to a 1e-3 which is ~1% of the smallest GDPs we now have with REMIND EU. It ensures that taxes do not create or eat money. The precision may be up for debate as there is a trade-off between longer run time and better tax convergence. In my test runs (see below) iterations increased, but only in one case up to 100 (SDP900).