If a SID hasn't started trading yet, pandas' convention is to use nans.
Before this change, zipline would raise an exception if there were nans in the
input data.
We now skip events where the prices contains a nan and has not been traded
before (in which case forward fill).
Fixes#446.
For more discussion see https://github.com/quantopian/zipline/pull/485.
Basically, 1000 is just a number that was supposed to be high enough if no volume was available. It turns out that number is actually very low so now we are increasing it so that volume restrictions should no matter. 1e9 of shares ought to be enough for anybody ;).
Thanks to @jlowin for pointing that out.
jfkirk