docs(quantity): add module-level docstrings to quantity package

autolens/quantity/fit_quantity.py

@@ -1,3 +1,20 @@
+"""
+Quantity fit class for comparing derived lensing quantities across different models.
+
+``FitQuantity`` extends the ``autogalaxy`` base class to accept a ``Tracer`` as the
+model object.  It fits a ``DatasetQuantity`` (e.g. a 2D convergence map, deflection
+angle map, or potential map) with the corresponding quantity derived from a tracer.
+
+Typical use cases include:
+
+- Comparing the convergence of a power-law mass profile with that of an NFW profile on
+  the same grid, to quantify how well one approximates the other.
+- Fitting deflection angles computed by one lens model to those of another model, to
+  propagate uncertainties from a parametric fit into a more complex model.
+
+The ``AnalysisQuantity`` class uses ``FitQuantity`` as its evaluation engine when
+performing a Bayesian model fit via a non-linear search.
+"""
 import autogalaxy as ag
 
 from autogalaxy.quantity.dataset_quantity import DatasetQuantity

autolens/quantity/model/analysis.py

@@ -1,3 +1,14 @@
+"""
+Analysis class for fitting a ``Tracer`` model to a derived lensing quantity dataset.
+
+``AnalysisQuantity`` combines the ``autogalaxy`` quantity analysis with
+``AnalysisLens`` so that a ``Tracer`` is used to evaluate the derived quantity
+(e.g. convergence, deflection angles, potential) being fitted.
+
+It is used for model comparison workflows where two different mass profile families
+(e.g. power-law vs. NFW) are compared by fitting one's predicted lensing quantity to
+the other's, with a noise-map providing the figure-of-merit.
+"""
 import numpy as np
 
 import autofit as af