Core im additions patch v2 (#146)

* Update core-im-source.yaml

Adding additional attributes and classes to the core-im-source file from the full/informtive model.

* Update core-im-source.yaml

Continued adding new classes and properties to the core-im.

* Update core-im-source.yaml

More additions to flesh out the v1 core-im.

* fix formatting errors

* Update core-im-source.yaml

- Fixed range/data type for `StudyResult.dataItems` property
- added an 'extends' element to `VariantClassification.classification` property

* Update core-im-source.yaml

Added an 'extends:derivedFrom' line to the StudyResult.sourceDataSet proeprty.

* core-im-source.yaml edits necesary to get the Make process to work

* updated sphinx version to bypass rtd build bug

* Update core-im-source.yaml (#131)

Updated free-text description of the StudyGroup class to provide more clarity.

---------

Co-authored-by: Matthew Brush <mhb120@gmail.com>
Co-authored-by: Alex H. Wagner, PhD <Alex.Wagner@nationwidechildrens.org>

schema/va-spec/core-im/core-im-source.yaml

@@ -459,8 +459,9 @@ $defs:
     maturity: draft
     description: >-
       A collection of individuals or specimens from the same taxonomic class, selected for analysis in a scientific 
-      study based on their exhibiting one or more common characteristics  (e.g. ethnicity, race, country of origin, 
-      clinical history, age, gender, geographic location, income, etc.)
+      study based on their exhibiting one or more common characteristics  (e.g. species, ethnicity, race, country of 
+      origin, clinical history, age, gender, geographic location, income, etc.) May be referred to as a 'cohort' or 
+      'population' in specific research settings.
     heritableProperties:
       memberCount:
         type: integer