WIP: Ensure all specs are validated (#526)

* Include data_type property on dataset and group specs

* Ref #507
* This property facilitates validating builders against specs

* Include additional validator unit tests

* Ref #507

* WIP Refactor GroupValidator and update logic to pass unit tests

* Ref #507
* Resolves the first two points of #507
* Many comments were added during refactoring to help with understanding
  these comments will be removed before the branch is ready to merge

* Add new unit tests for the validator

* Ref #507
* These unit tests are not currently passing but should by by the end of this issue

* Refactor GroupValidator and update matching between builder and spec

* Ref #507
* Now passes all tests
* The code will be further refactored and clean up before it is ready

* Refactor GroupValidator and SpecMatcher and add documentation

* Ref #507

* Clean up extraneous comments

* Refactor out IllegalLinkError creation to be consistent with other error creation

* Update changelog

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Bug fixes
 - Fix CI testing on Python 3.9. @rly (#523)
+- Fix certain edge cases where `GroupValidator` would not validate all of the child groups or datasets
+  attached to a `GroupBuilder`. @dsleiter (#526)
 
 ## HDMF 2.4.0 (February 23, 2021)
 

src/hdmf/spec/spec.py

@@ -483,14 +483,19 @@ def def_key(cls):
 
     @property
     def data_type_inc(self):
-        ''' The data type of this specification '''
+        ''' The data type this specification inherits '''
         return self.get(self.inc_key())
 
     @property
     def data_type_def(self):
         ''' The data type this specification defines '''
         return self.get(self.def_key(), None)
 
+    @property
+    def data_type(self):
+        ''' The data type of this specification '''
+        return self.data_type_def or self.data_type_inc
+
     @property
     def quantity(self):
         ''' The number of times the object being specified should be present '''