MAINT: More precise extraction of returns from returns container.

Use slice to date, `[:dt]` instead of `pd.Series.valid` to extract
from returns containers.

Using `valid` lead to some confusion when debugging tests, because
it papers over missing data.

The use of `.valid` was based on the assumption that all values
from the zeroth date to the current algo date are populated,
with no trailing values.
`[:dt]` extracts the same data, but in a hopefully more precise
and explicit fashion.