new endpoint : combo sensitivity analysis

[yellowbean]

Currently, user can perform sensitivity on a single component, either on Deal Obj or Deal Run Assumption or Pool Performance assumption.

But there is not functionality to expose a Joint sensitivity that simulation both Deal Obj AND Pool Performance Assumption, i.e: Different tranche sizing with different pool performance.

Then the input signature :

A map of deal obj, A map of pool performance, A map of Deal run assumption.

The response type:

A map with 
* Key : (name of deal obj, name of pool performance, name of deal run assump)
* Value : <Single Run Result>