Yield Curve Comparer Improvements

[amuolo]

The yield curve comparer needs improvement. For example importing the same YC for 2 different months is not required.

[calderinisystemorph]

Comparing Yield Curve for different Month is considered redundant since it is defined with yearly base. Nevertheless, different Yield Curves may differ for Month and Value: in this case we want to save and use the most recent one.