Compute max-profit over all forms if "form" is None

[hanase]

This was a bug: If the argument "form" is None, the max profit should be computed over all forms.

When calling the function with form=None, the original code failed at the line df = df[df.max_profit_far > 0], since df.max_profit_far does not exist.

[coveralls]

Coverage increased (+0.04%) to 94.335% when pulling d563ce7 on hanase:max-profit-fix into d9a058b on UDST:master.

[Eh2406]

Good Catch! Maybe we should add a test to keep this from breaking again?