add support for multiple Experiment() #89

Robustify multiples launches of researcher as reported in #89 (closed) #72 (closed) :

• in notebook
• in script

Supported scenario (tested with getting_started.py):

• multiple successive launch in notebook without a clear output and restart between launches : create Experiment(), exp.run(), create Experiment(), exp.run(), etc. (or relaunch without re-create) This scenario means multiple Experiment() existing in a notebook
• multiple Experiment() objects in a script: run a script that does create Experiment(), exp.run(), create Experiment(), exp.run(), etc.
• launch in notebook, then launch a script
• launch a script, then launch a notebook

Scenario not intended to be supported by this branch:

• launch 2 nodes simultaneously with same node_id: not detected, may not work correctly
• launch 2 researcher simultaneously with same node_id: appears to work in simple cases, but probably needs robustification of message handling in researcher

Added in the branch:

• singleton for Requests() to avoid conflicting instances in same program
• randomization of MQTT client ID done by broker (not Messaging)
• unique queue per Requests() instance to avoid conflict between several programs running for same {node,researcher}_id on a host
• some basic error handling and error messages for communication layers
• also handles the case where 2 instances exist with same {node,researcher}_id: marginal for the finally retained scenario with random client_id (detect very rare case of random id conflict for MQTT connection), mandatory for alternative scenario (MQTT connection using the {node,researcher}_id as client_id)