Add a Clean option

[kent0]

I think a clean option that removes everything inside runs/ will be convenient.

[thilinarmtb]

This would be a great addition !!

[thilinarmtb]

Since we have all the data under runs/<machine_name>, may be we can have options like:

./go.sh clean all -- Deletes everything under runs/
./go.sh clean linux -- Deletes runs/linux if exist
./go.sh clean linux theta - Deletes run/linux and runs/theta if exist