Use multiprocessing Pool.map for execute_parallel_tasks
Use multiprocessing Pool.map for execute_parallel_tasks

Fixes #512

bearecinos/oggm,TimoRoth/oggm,OGGM/oggm,bearecinos/oggm,anoukvlug/oggm,OGGM/oggm,juliaeis/oggm,TimoRoth/oggm,juliaeis/oggm,anoukvlug/oggm