Implement Block() class

[capsulecorplab]

Implement attributes, constructors, and methods for Block() class.

Attributes:

• parts - dictionary of blocks that are owned/contained within the block instance.
• values - dictionary of value properties that are of type int or float, or has attribute 'unit' (thus, can be a UnitRegistry from pint package)
• references - dictionary of reference properties (WIP)
• flows - dictionary of flow properties (WIP)
• constraints - dictionary of constraint properties that are of type Constraint
• uuid - universally unique id
• multiplicity - int

Methods:

• add_part()
• remove_part()
• add_value()
• remove_value()
• add_reference()
• remove_reference()
• add_flow()
• remove_flow()
• add_constraint()
• remove_constraint()