Allow Sections to have multiple ClassPeriodReferences

Summary of outcomes for Data Standard 2.1

• Problem Addressed: Some sections may have multiple meeting times but the current model only allows for one ClassPeriod reference per Section.
• High-level Reasoning: Some sections, like labs or double-block scheduling, may meet over multiple class periods. The Section domain entity previously only allowed for one ClassPeriod reference as part of the identity. However, this field is being made optional as part of an effort to make the key less volatile (DATASTD-891) and may now be referenced as a collection to accommodate multiple ClassPeriod references.
• Changes Made: Changed the domain entity ClassPeriod reference in the Section domain entity to optional collection

Original ticket description follows:

As described in DATASTD-586, it is possible that a Section could have multiple meeting times during a day. Investigate making a collection.