MAINT: Slice into returns containers instead of using .valid()

The slicing syntax is more explicit about declaring:
'get all returns up until the current dt'.

Also, protects against NaNs that occur before the current dt
being silently ignored.
i.e. the *_returns_cont series *should* have values from start
to current dt, but the .valid() call was occluding a bug where
it wasn't.