When the prev_close is 0 or does not exist, the resulting ration was either +inf
or nan, respectively.
Create a mask on the non-zero effective dates, where effective date is only
written when the prev close is sufficient for a valid ratio; and use that mask
to filter out the bad rows.
Also, use prev close as the effective date.
To prepare for querying for payouts from SQLite, write the dividend
payouts to a new table `dividend_payouts`.
Change the expected columns of the passed dividend frame to contain the
payout data, and use that data to calculate the ratios (this moves
internal code that was calcualting the ratios into Zipline.)
The end result is that instead of just a `dividends` table with the
backward looking adjustment ratios, also write a `dividend_payouts`
table and a `stock_dividend_payout` table.
Put the logic for reading and writing the equity price and adjustment
data into a module located in data, making it distinct from the pipeline
loader usage of the formats.
This prepares for both incoming changes of how adjustments are written,
(which includes using the bcolz daily reader as an input), as well as
eventually providing the readers to a DataPortal object.
Thomas Wiecki