Add changes to run_exp.py to integrate NeST with aiocoap

• Changes in experiment/run_exp.py:

  Add new tool 'coap', with dependency check for aiocoap handled by the is_package_installed in engine/util.py.

  Add function setup_coap_runners() for creating CoAPRunner objects. This function is called from run_experiment(). This also runs server processes whenever necessary.

  Make changes to setup_parser_workers(), dump_json_outputs() and cleanup() to include CoAP relevant procedures.

  Add a check to get_dependency_status() to call is_package_installed() to check for aiocoap in the case of CoAP.

• Changes in engine/util.py: Add function is_package_installed() for checking if a given python package is installed. It uses the sys package to call sys.modules() and checks if the given package is in that list.