ADR: DiffusionModel

[henrikjacobsenfys]

This ADR documents the design decisions behind the DiffusionModel

DiffusionModel consist of a base class, DiffusionBase and a class for each DiffusionModel. Currently, only two diffusion models exist, but more will be added.

The base class has unit (for the energy axis) and scale.

All diffusion models also have the following methods:

• calculate_width: calculates the width of the Lorentzian(s) in the model and returns it as an np.ndarray. If multiple Lorentzians, then it will be a list of arrays.
• calculate_EISF: calculates the elastic incoherent structure factor, i.e. the area of the elastic signal
• calculate_QISF: the quasielastic incoherent structure factor, i.e. the area of the Lorentzian(s).
• create_component_collections: Takes Q as input and creates a ComponentCollection for each Q. Each collection has the appropriate delta functions and Lorentzians, with Parameters depending on Q and e.g. scale and diffusion coefficients as appropriate.
• _write_width_dependency_expression: helper function to write the dependency expression for the width of the Lorentzian
• _write_width_dependency_map_expression: same, but for the map
• _write_area_dependency_expression and _write_area_dependency_map_expression: same as above but for the area

To handle units we also use DescriptorNumber representations of hbar and Ångstrom