'debug' module

This merge request adds a new debug module in bliss.common.

The debug module has 2 public functions:

• trace(on=True, output_file=None, file_mode="wa")
  • if on is True, uses Python profiling hook to set a profiling function that prints elapsed time, filename, line number for any subsequent execution for code within the BLISS project itself
  • an optional output_file can be given, in this case stdout is also duplicated to this file for easier debugging (trace + output)
  • output file mode can be specified, by default the output file grows
  • calling .trace(False) removes the trace function
• step_into(func, *args, **kwargs)
  • calls function func with arguments
  • enters pdb at the first line of func
  • solves #271 (closed)

About trace(): the idea here is to have a generic trace function for internal BLISS calls, but it would be good to solve #368 (closed) as well.