Add opal.core.enum docs and fix a number of renaming bugs in our docs

doc/docs/guides/components_overview.md

@@ -13,7 +13,7 @@ It contains a core [data model](datamodel.md) for persisting clinical informatio
 as well core clinical archetypes out of the box that build on many
 years of collaboratively modelling clinical concepts by medical domain experts.
 
-Opal also makes it easy to integrate standard clinical [reference data](lookup_lists.md)
+Opal also makes it easy to integrate standard clinical [reference data](referencedata.md)
 into your application - things like medical diagnoses, tests, drugs et cetera.
 
 Some common functional components end up in to most clinical applications, and many

doc/docs/guides/datamodel.md

@@ -63,4 +63,4 @@ Opal comes with a set of clinical terminology data models out of the box. - we o
 want to link our records to one of these - for example to record a type of condition
 a patient might have, or a drug they are taking.
 
-Full documentation of these is available in the [Lookup lists](lookup_lists.md) documentation.
+Full documentation of these is available in the [Reference Data](referencedata.md) documentation.

doc/docs/reference/core_fields.md

@@ -0,0 +1,34 @@
+# opal.core.fields
+
+The `opal.core.fields` module contains helper functions for working
+with fields, as well as custom Opal field definitions.
+
+## is_numeric
+
+A helper function that returns true if a field is numeric. For our
+purposes, Integers, Decimals, BigIntegers, Floats and Positive Integers
+are considered numeric.
+
+```python
+is_numeric(models.IntegerField())
+# -> True
+```
+
+
+## enum
+
+A helper function that returns a Django choices definition from star args.
+
+```Python
+enum('one', '2', 'III')
+# -> (
+#     ('one', 'one'),
+#     ('2', '2'),
+#     ('III', 'III')
+#    )
+```
+
+## ForeignKeyOrFreeText
+
+A field that stores it's value as a generic foreign key to an Opal LookupList
+or as the value in a CharField.

doc/docs/reference/reference_guides.md

@@ -21,7 +21,8 @@ The following reference guides are available:
 [opal.core.patient_lists](patient_list.md)|Patient Lists - defining different types of list|
 [opal.core.plugin](plugin.md)| Plugins - defining plugins to package reusable functionality
 [opal.core.menus](core_menus.md)| Menus - declaring application menus
-[opal.core.log](core_log.md)| Log Helpers - custom email error loggers
+[opal.core.log](loggers.md)| Log Helpers - custom email error loggers
+[opal.core.fields](core_fields)| Field helpers - custom field types and utility functions|
 
 ### Angular Services
 |

doc/mkdocs.yml

@@ -65,6 +65,8 @@ pages:
       - OpalPlugin: reference/plugin.md
       - Core.Views: reference/core_views.md
       - Menus: reference/core_menus.md
+      - Log: reference/loggers.md
+      - Fields: reference/core_fields.md
 
       # Angular Services
       - reference/javascript/patient_service.md