This pull request adds support for including constraint property metadata in statistical observation DataFrames and introduces new models and endpoints for handling constraint properties associated with statistical variables. The changes are primarily focused on enhancing the ability to fetch and display constraint information related to statistical variables in the DataFrame output.

Enhancements to DataFrame constraint metadata:

• Added an include_constraints_metadata parameter to the observations_dataframe method in client.py, allowing users to optionally include constraint property DCIDs and names in the returned DataFrame. [1] [2] [3]
• Implemented the add_property_constraints_to_observations_dataframe utility function, which populates constraint property DCIDs and names for each observation in the DataFrame.

New models and endpoint methods for constraint properties:

• Introduced StatVarConstraint and StatVarConstraints models in models/node.py to represent constraint properties and their values for statistical variables.
• Added methods to NodeEndpoint in endpoints/node.py for fetching constraint property DCIDs and names (_fetch_property_id_names) and for retrieving constraint property/value pairs for statistical variables (fetch_statvar_constraints).

Internal import updates:

• Updated imports in client.py and endpoints/node.py to include new utility functions and models required for constraint property handling. [1] [2]