shift technology cost convergence to 2070 and increase coal power plant convergence target costs from 1260 to 2000$(2015)/kW

Created by: robertpietzcker

Reason: coal power plant investment costs were reduced during the Jan/2019 validation of REMIND below the value found in literature. Current cost estimates in developed countries are in the range of 2000-3600$/kW, so the old implementation meant substantial cost reductions, which are not expected in literature. To prevent a very fast increase of costs in developing countries, the convergence point was shifted to 2070.