[Enhancement] Add Multi-Country Support & ECB Yield Curves To fixedincome.goverment.yield_curve(provider='econdb')

[deeleeramone]

1. Why?:

   • Added functionality to fit requirements of @andrewkenreich and @minhhoang1023.

2. What?:

   • In, obb.fixedincome.goverment.yield_curve(provider='econdb'), multiple items allowed for:
     • 'country'
     • 'date'
   • Adds a @computed_field to the output, maturity_years, which is the decimal representation of years-to-maturity.
     • It assumes that the maturity field is formatted as: 'month_X', 'year_X', or 'year_X_month_X'. If not, computes as None

3. Impact:

   • Improved data coverage and enhanced user experience.

4. Testing Done:

[IgorWounds]

What do you think about this?

parts = self.maturity.split("_")
    months = sum(
        int(parts[i + 1]) * (12 if parts[i] == "year" else 1)
        for i in range(0, len(parts), 2)
    )
    return months / 12