Changed default function to command_print, added firingLimit=1 for each process.
Implemented function "command" which excutes an external process and waits
for its completion. Added new function "command_print" which only prints
the command that would be executed.
Example emulated Montage workflows now use "command_print" function.
Processes in Montage_143 have added "firingLimit=1" option.
Input signals now have "sigIdx" property which indicates the sequential
number of the signal instance.
New features added to the Dataflow process (to be added to other types):
- Concurrent processing of multiple firings; parameter 'parlevel'
  indicates how many firings can run concurrently. See workflow
  "Wf_parlevel_test.json" for an example.
- Added firing limit, set by the "firingLimit" property.
  When the process fires the number of times specified by the firing
  limit, it automatically finishes the execution and accepts no more
  signals.
- Signals emitted by the processes now have additional attributes:
  * "source": the id of the process which produced the signal
  * "firingId": the sequential number of the firing in which the signal was
  produced.
An example workflow which demonstrates the concurrent processing of
process firings, configured by the 'parlevel' property.