change tax convergence criterion for nash runs such that tax revenues need to be below one percent GDP

Created by: fschreyer

Change tax convergence criterion for nash runs. Now tax revenues in all regions and time steps need to be below one percent of GDP. In a suite of SSP2 and SDP standard test runs, this did not significantly increase the number of iterations relative to the version without tax convergence check.

Would be nice if sombody from RSE could add this criterion to the rs script. So that if p80_messageShow("taxconv") = YES, i.e. taxes did not converge, this also shows up in the rs.