Parratt Data Structures

Here are the data structures with brief descriptions:

_data	This struct holds the data to be fit
_layer	This struct holds layer-specific information
_method	This struct allows the choice of the fit method
_misc	This struct holds miscellaneous data
_model	This struct holds the adjustable model parameters
_other	This struct holds static, model parameters
_parratt	The container struct for all data and parameters

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