Toolbox: Single processor execution

[guiguem]

As currently implemented it is not possible to run a config file with a single processor as:

Traceback (most recent call last):
  File "/Users/guig098/Work/Project8/python_environments/morpho2/bin/morpho", line 64, in <module>
    myToolBox.Run()
  File "/Users/guig098/Work/Project8/python_environments/morpho2/lib/python3.7/site-packages/morpho/utilities/toolbox.py", line 156, in Run
    if not self._DefineChain():
  File "/Users/guig098/Work/Project8/python_environments/morpho2/lib/python3.7/site-packages/morpho/utilities/toolbox.py", line 115, in _DefineChain
    for a_connection in self.config_dict['processors-toolbox']['connections']:
TypeError: 'NoneType' object is not iterable