MAINT: Move search day distance function to module level.
MAINT: Move get_treasury_rate to risk module level.
MAINT: Factor out start and end date choose_treasury parameters.

Preparing for move of method to a module level function.
MAINT: Remove saving of treasury duration.

The treasury_duration member in RiskMetrics is never used except
for in unit tests.

Remove the saving of treasury_duration in preparation for the
move of the choose_treasury method out of the RiskMetrics classes.

Down the line, if we do restore the sanving of treasury_duration,
choose_treasury can return a tuple that includes treasury_duration
instead of just returning the rate.
MAINT: Factor out treasury_curve argument from risk choose_benchmark

Move the reference to self.treasury_curve to a parameter, on the path
of making this method a module level function.
MAINT: Move choose_treasury method from risk metrics class to module.
MAINT: Refactors out selection of treasury duration its own function.

Instead of having the duration selection logic in choose_treasury,
break out duration specific logig into another function.
MAINT: Mask treasury_curves used by risk metrics to period range.

So that calculations that leverage the range of the treasury_curves,
like `pd.Series.searchsorted` will not overshoot the 'end' of the
range we are calculating risk metrics.
MAINT: Use searchsorted instead of bisect on treasury curves.

Remove use of .keys() and creation of a new list of the curve
Series's values.
Merge move of risk module treasury curve methods to module functions.

Intent is to both make the inputs to the treasury curve logic
more clear/explicit, as well as reducing the responsibilities
of the RiskMetrics classes.

Functions to move:
- search_day_distance
- get_treasury_rate
- choose_treasury

Also add new function:
- select_treasury_duration, split out of choose_treasury