feat(apis_entities): implement default abstract entity classes

Closes: #403

apis_core/apis_entities/abc.py

@@ -0,0 +1,35 @@
+from django.db import models
+from apis_core.core.abc import LabelBaseModel
+
+#########################
+# Abstract base classes #
+#########################
+
+
+# These abstract base classes are named after
+# CIDOC CRM entities, but we are *NOT*(!)
+# trying to implement CIDOC CRM in Django.
+
+
+class E21_Person(LabelBaseModel, models.Model):
+    forename = models.CharField(blank=True, default="")
+    surname = models.CharField(blank=True, default="")
+    gender = models.CharField(blank=True, default="")
+    date_of_birth = models.DateField(blank=True, null=True)
+    date_of_death = models.DateField(blank=True, null=True)
+
+    class Meta:
+        abstract = True
+
+
+class E53_Place(LabelBaseModel, models.Model):
+    longitude = models.FloatField(blank=True, null=True)
+    latitude = models.FloatField(blank=True, null=True)
+
+    class Meta:
+        abstract = True
+
+
+class E74_Group(LabelBaseModel, models.Model):
+    class Meta:
+        abstract = True

apis_core/core/abc.py

@@ -0,0 +1,8 @@
+from django.db import models
+
+
+class LabelBaseModel(models.Model):
+    label = models.CharField(blank=True, default="")
+
+    class Meta:
+        abstract = True