MAINT: Use metadata method for reader init.
MAINT: Use metadata method for reader init.

Use the preexisting metadata method when instantiating the minute bar
reader.

An internal sublcass uses the `_get_metadata` method to setup data for
directories that have not used the new writer/reader interface.
(i.e. allows for reader creation when the metadata.json file does not
exist.)

umuzungu/zipline,wilsonkichoi/zipline,florentchandelier/zipline,humdings/zipline,alphaBenj/zipline,enigmampc/catalyst,quantopian/zipline,Scapogo/zipline,florentchandelier/zipline,nborggren/zipline,bartosh/zipline,bartosh/zipline,Scapogo/zipline,nborggren/zipline,magne-max/zipline-ja,wilsonkichoi/zipline,humdings/zipline,alphaBenj/zipline,magne-max/zipline-ja,enigmampc/catalyst,umuzungu/zipline,quantopian/zipline