ENH: Makes breakpoints work in algoscripts started with the run_algo.py
script.

Adds an option kwarg to TradingAlgorithm named 'algo_filename' that
represents the file where the algoscript came from (if any). The
run_algo.py script will pass this argument with the value passed to the
'-f' flag. The default name is '<string>' to represent that the script
is coming from a string in python and not a file. This matches the
behavior of exec and the python convention for compiling code objects.
STY: Removes unused import in run_algo.py