The daily/session bar reader's `spot_price` took the same parameters and
returned the same kind of output as the minute bar reader's `get_value`.
Standardize on one method to make a common interface, which may be
formally factored out in a later patch; to help enable writing reader
implementations or mixins which can be agnostic to the bar frequency.
Replace `DailyBarReader` with `SessionBarReader`.
This was intended to go with the patch that added the `SessionBarReader`
abstract base class.
Also, added `trading_calendar` property decorator.
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.
Eddie Hebert