Lots more Document data definition

Schedule updates:

* Defined Schedule types
* Updated schedule loader to validate with pydantic
* Added ability to specify predecessor type and lead/lag

Other structural/outline stuff as well

Oh and added a unit test.

documents/project/schedule.rst

@@ -0,0 +1,12 @@
+.. raw:: latex
+
+    \begin{landscape}
+
+Milestone Schedule
+##################
+
+.. schedule:: _data/schedule.yaml
+
+.. raw:: latex
+
+    \end{landscape}