Add possibility to run in parallel or in series the `downscale_climate()`

[ArcticSnow]

Include a possibility to not run topo_scale in parallel.

• [ ] add an if statement in downscale_climate()

  if run_in_parallel:
  do pooling
  else:
  for loop

• [ ] add a flag in the config file.

[ArcticSnow]

Indicating n_cores: 1 in config file does the trick!