Multisite support

[yadudoc]

This is to track ongoing work on the multisite branch.

Why is this important:

1. This will allow us to manage apps with different resource requirements in a single workflow, such as apps that only need a core and should be executed on the login node vs, apps that run on multiple
   nodes with MPI.
2. Light weight apps executing on threads could perform flowcontrol and launch heavy tasks.

• Accept multiple executors in the dfk
• Launch multiple executors in the DFK from the config definition
• Support for multiple IPP executors
• Attach specific tasks to sites via enhancements to the decorator to take sites kwarg :

@App ('bash', dfk, sites=['local'])
def foo (x):
    return 'echo $(({0} * 2))

@App ('bash', dfk, sites='all' ) # Sites = 'all is the default
def sleep_compute (x) :
    return 'sleep x'

[yadudoc]

Here's a test from the test_suite that shows how all of this would work :
https://github.com/Parsl/parsl/blob/multisite/parsl/tests/test_scaling/local_multisite/test_ipp_None_local.py