Show how to use lookup tables with obs models

This uses the epifx.obs.PopnCounts observation model, which allows the
observation probability to be defined in a lookup table.

doc/how-to/index.rst

@@ -17,9 +17,51 @@ Defining prior distributions
 Provide models with lookup tables
 ---------------------------------
 
+Provide observation models with lookup tables
+---------------------------------------------
+
+An observation model **may** allow some of its parameters to be defined in
+lookup tables, rather than as fixed values.
+This allows parameters to differ between particles and to vary over time.
+
+.. note:: The observation model must support using lookup tables for parameter
+   values.
+   The example shown here uses the ``epifx.obs.PopnCounts`` observation model,
+   which allows the observation probability to be defined in a lookup table.
+
+To use a lookup table for an observation model parameter, the scenario must:
+
+* Define the lookup table by giving it a name (in this example, ``"pr_obs"``)
+  and identifying the data file to use (in this example, ``"pr-obs.ssv"``);
+* Notify pypfilt_ that each particle should be associated with a column from
+  this lookup table, by adding the table's name to the
+  ``"sample_lookup_tables"`` list; and
+* Notify the observation model that it should use this lookup table for the
+  observation probability, by providing it as an argument to the observation
+  model's constructor (in this example, the argument name is
+  ``"pr_obs_lookup"``).
+
+These steps are illustrated in the following TOML_ excerpt, which shows only
+the relevant lines:
+
+.. code:: toml
+
+   [scenario.test]
+   sample_lookup_tables = ["pr_obs"]
+
+   [scenario.test.lookup_tables]
+   pr_obs = "pr-obs.ssv"
+
+   [scenario.test.observations.cases]
+   model = "epifx.obs.PopnCounts"
+   init.pr_obs_lookup = "pr_obs"
+
 Using lookup tables in your own models
 --------------------------------------
 
+Using lookup tables in your own observation models
+--------------------------------------------------
+
 Defining and using parameters
 -----------------------------