Add a `research_notes` property to base property class

[gknapp1]

Currently there is a file-level notes field, but it could be helpful to have a property-level note field as well to document property-level assumptions or specifics if needed. Currently doable to document property-level notes in a long file-level notes field, but this can get awkward to write in the JSON format and does not explicitly connect the assumption to the associated property.

Any research notes could be added to the write_markdown output.