In preparation for making a resampling session bar reader, make an
abstract base class with the methods currently used by consumers of the
reader; which should assist in making a drop-in replacement of the daily
bar reader.
While pulling out the interface, it does appear that `spot_price` and
the minute bar reader's `get_value` are the same method but by different
names, showing that there may be room for having both `MinuteBarReader`
and `SessionBarReader` sharing a common `BarReader` interface.
