Charm Pool Examples: Add check for more than 1 PE #263

Description: Running locally on my Mac with a python virtual environment, the charm pool examples and some of the Ray examples start with a default of 1 PE. This leads to error messages stating that charm.pool should run with more than 1 PE.

Example:

(charmpp_env) python3 ./examples/pool/pool_simple.py

Error message:

site-packages/charm4py/pool.py", line 105, in __start__
    assert self.num_workers > 0, 'Run with more than 1 PE to use charm.pool'
------------- Processor 0 Exiting: Called CmiAbort ------------
Reason: AssertionError: Run with more than 1 PE to use charm.pool

Possible Solution:
In the examples, add a check for `charm.numPes() >1'. If the check fails, it would be helpful to point the user to the documentation on how to adjust the number of processes.