Allow users to overwrite configs

[amanmajid]

Currently, users will load and solve a model as follows:

from tz_pypsa.model import Model
n = Model.load_model('ASEAN')
n.optimize()

Users cannot overwrite model configurations unless they edit the raw input files. We should give users the ability to overwrite configs. For example, something like:

from tz_pypsa.model import Model

update_config = {
   'global_constraints' : {
           'annual_co2_budget': false,
           'bus_self_sufficiency': true,
           'cumulative_p_nom_max': true,
   }
}

n = Model.load_model('ASEAN', update_config=update_config)

n.optimize()