Rujie

I have implemented use case 1. The modifications are:

1. In ems.py: created new class ems_constantoutput, new function ems_cplex_constantoutput, and new function ems_cplex_parts_constantoutput
2. Created new hpp_assembly_constantoutput to import ems_constantoutput
3. Added one column in ./examples/Europe/2030-EL_PRICE.csv for PPA price
4. Added input "load_min" in ./examples/Europe/hpp_pars.yml (it is not used)