[FEATURE] adapt one date format

[jpdeleon]

fastquant module uses "%Y-%m-%d" date format whereas disclosures uses "%m-%d-%Y". We should adapt only one format and I prefer the second one because it is more natural.

Note however that datetime.datetime used in several parts of the code adapts the prior format.

[enzoampil]

ahhhh I actually find the first one to be more common :( Although, don't feel super strongly about it so don't mind either implementation.

Do note that our readme has used "%Y-%m-%d" for while so our users are probably more used to it by now.

[jpdeleon]

Ok, let’s adapt the first format. It be great to implement #80 to update notebooks automatically.