FL-1273 Add demonstration of look-through into underlying constituents

[steco]

Pull Request Checklist

• [ ] Changes follow the style guide
• [ ] Tests pass
• [ ] Raised the PR against the develop branch
• [ ] Notebook outputs do not contain any priviledged data

Description of the PR

Added a section which demonstrates the LUSID API call to retrieve the fully-expanded reference portfolio, ie using look-through

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