Introduce AssessmentPeriod Common Type

Summary of outcomes for Data Standard 3.0

• Problem Addressed: Prior to v3.0, the AssessmentPeriod descriptor contained an optional BeginDate and EndDate. These dates included in a descriptor create some confusion, as they appear to be providing some metadata about the descriptor values when there are already effective begin and end dates included. Additionally, in v3.0, any descriptors containing additional fields are remodeled to only contain the standard descriptor fields.
• High-level Reasoning: The BeginDate and EndDate that were previously included in the AssessmentPeriod descriptor were actually intended to communicate the begin and end dates for an assessment period - not the descriptor values themselves. By creating an AssessmentPeriod common type to contain the AssessmentPeriod descriptor, BeginDate and EndDate may be included in the common type instead. This still provides the BeginDate and EndDate for an Assessment's assessment period AND now allows multiple assessments to use the same AssessmentPeriod, even if the BeginDate and EndDate for that assessment period are not the same across all assessments.
• Changes Made: A new AssessmentPeriod common type was created (to contain the AssessmentPeriod descriptor) and referenced from the Assessment domain entity as optional, instead of a direct reference to the AssessmentPeriod descriptor. The BeginDate and EndDate were removed from the AssessmentPeriod descriptor and added to the new AssessmentPeriod common type as optional properties.

Original ticket description follows:

The AssessmentPeriod descriptor is currently serving more as a common type. Instead, create an AssessmentPeriod common type to reference the AssessmentPeriod descriptor and optional BeginDate and EndDate.