Support on "back contracts" of continuous futures

[LouisSzeto]

Expected Behavior

using contractDepthOffset = n could get both front and back contracts in particular.

Actual Behavior

https://www.quantconnect.com/forum/discussion/12644/continuous-futures-support/p1/comment-39158 The maximum contract depth is unknown, so back contract cannot be fetched.

Potential Solution

Provide MaximumContractDepth attribute on continuous futures, or allow contractDepthOffset = -1, etc.

Reproducing the Problem

N/A

System Information

N/A

Checklist

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• [ ] I have provided detailed steps to reproduce the issue