documentation for references and restructuring documentation

documentation/entities/time_unit.md

@@ -0,0 +1,16 @@
+### Time Unit
+
+A Time Unit can be given the following information:
+
+- Name (`tu_display_name`)
+- A Rank (`rank`)
+- A comment (`tu_comment`)
+- Sequence (`sequence`)
+- Lower Bound Id (`low_bnd`)
+- Upper Bound Id (`up_bnd`)
+
+Out of these, the rank and comment fields are optional and the rest are required.
+
+The components that are used to change the upper or lower time bounds are `TimeBoundSelection` components. These are almost exactly the same as `FieldWithTableSelection` components, but when their value is changed, they also update the `up_bound` and `low_bound` objects in `editData`. This way when the upper/lower bound id's are changed by the user, the actual bounds are immediately updated too and can be shown to the user or passed to a validator. `TimeBoundSelection` components should not be used anywhere else.
+
+After the time unit is created, its ID (`tu_name`) is set to be the same as its name, but lowercase (for example, a time unit with the name "Abdounian" has the id "abdounian"). Because of this, changing an existing time unit's name causes a foreign key error in the database because the "now_time_unit" and "now_tau" tables are connected by it. When editing a time unit, the name field is currently disabled to not allow this error to happen.