batch_run should default to running on 1 CPU

[rht]

Currently, it uses up all the CPUs available in a system. This causes complication with the tutorial for Windows, where a nasty workaround is needed for multiprocessing (i.e. the freeze_support).

Then, if number_processes is > 1 and on Windows, we show a warning that tells user to add freeze_support() (we pre-emptively tell the user of this issue instead of waiting for it to happen). This warning can be disabled via a flag to the function:

batch_run(
    ...
   hide_windows_freeze_support_warning=True,
   ...
)

Originally posted in #1202.