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
BUG: When increasing the length dynamically, the rolling panel was
getting filled with the wrong datetimes and causing errors.

Updates the logic for addressing missing datetimes and adds unit tests
for the 2 main cases (no missing datetimes, and some missing datetimes).