Add Support for Custom Market Data in Black-Litterman Model

[coderabbitai]

Description

Currently, the Black-Litterman model implementation doesn't support custom market data. When attempting to use custom market data, the following exception is raised:

raise Exception("Custom market data not supported at present")

This functionality would be valuable for users who want to provide their own market data rather than rely solely on DefiLlama.

Proposed Solution

1. Update the BlackLittermanYieldModel to accept custom market data
2. Validate the format of custom market data
3. Update documentation to explain how to provide custom market data

Related PR

This issue was identified during review of PR #37.

Issue opened by @coderabbitai on behalf of @JustAGhosT

[JustAGhosT]

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