CFA and SEM with PyMC (#698)

* First commit CFA and SEM

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* test pre-commit hooks

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* updated for pre-commit

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* updated git index

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* update with full run

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* add some text write up and a pair plot

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* update with full run and text

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* Run with explained variance calculations

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* Add sem structure section

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* full run with lavaan syntax noted

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* update with draft conclusion

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* tigher writing

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* tidying

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* emphasis on exchangeability

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* update plot sizes

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* add pearl reference

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* updating with Chris's comments and better parameterisation

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Author: NathanielF