DEV: Handles empty stash case in pre-commit hook.

Handles the case when reword is called and the `commit --amend` invokes
the pre-commit hook, when there is no stash the `git stash pop -q`
in that case leaves the rebase in an odd state.
Fixes by only calling the stash pop if changes were saved.
MAINT: Use numpy for risk period returns.

Following the lead of the RiskMetricsBatch conversion to use
more pandas and numpy.
Bringing the iterative and batch versions closer together as we
work towards folding them into one.