Currency Improvements

[ssanderson]

This PR contains several improvements to the currency conversion support that was added in #2586 and #2597.

• Fixes a rather embarrasing bug where we loaded currency codes incorrectly unless we queried for all the sids contained in an HDF5 file: fb07353.
• Adds handling for currency queries containing unknown sids. This is useful for robustness against invalid input data, or for against data artifacts produced at different times: 006b7b9. This works as follows:
  • Adds a new special sentinel, zipline.currency.MISSING_CURRENCY_CODE (value 'XXX', which denotes no currency according to wikipedia). This currency is returned by CurrencyAwareDailyBarReader for unknown sids, or sids for which currency is unknown. I'm using this rather than None or '\0\0\0' because we're currently returning codes in numpy arrays of dtype S3, which can't hold None, and these values might get displayed to users, which makes \0\0\0 an awkward choice. Another option would be to use None, and switch to using object arrays, but that makes things like sorting annoying (None can't be compared with strings in python 3). Object array with None is probably what we'd want to expose to users if we allow querying for currencies though (it's what we do elsewhere for string-typed data, at least), so that might be preferable.
  • Adds handling for queries of unknown currencies to the currency reader interface. Queries for unknown currencies yield np.nan.
• Adds version metadata and checking to the h5 currency format, to allow for safer future upgrades: 9e3deab.

[coveralls]

Coverage decreased (-0.002%) to 88.031% when pulling 09fb188 on currency-fixes into 5e61943 on master.

[peterhbromley]

This all LGTM, I like that you found the wikipedia reference that 'XXX' denotes missing currency. That makes me feel good about choosing that as the display value to the users.