Abstract classes

The plugin's Configuration definition includes a number of "abstract" classes of fields.

These classes are not referenced directly in the TOML syntax, but they form the basis for many of the properties available for parsing by the plugin.

Most items in AQTS field visit data support some form of "Comment" field, and that is what the Core abstract class provides.

Most items (Readings, Calibrations, Inspections, GageHeightMeasurements, and LevelSurveyMeasurements) also represent an activity that occurred at a specific moment in time, and that is what the Activity abstract class provides.

Some items (Visit, AdcpDischarges, and PanelDischargeSummaries) occur over a range of time, with a start and end time. This is what the TimeRangeActivity abstract class provides.