Node property storage and querying

[alvaradoo]

This commit involves new functionality and a few fixes:

• The new functionality involves being able to convert an Arkouda DataFrame to a property graph by calling the PropGraph.add_node_properties() method. This creates a two-dimensional array at the back end that stores arrays of variable size for each column. The columns of the two-dimensional array in the back-end corresponds to the data types that the property graph can store.
• To be able to store maps and two-dimensional arrays in the symbol table we generated two new symbol entries: SymEntry2D and MapSymEntry.
• Instead of requiring the user to name their label and relationship dataframe columns "nodeIds" and "src" and "dst" for adding in labels and relationships, we just assume the first column are node ids for adding in labels, and the first two columns for adding in edges.
• Some minor changes to documentation.

[zhihuidu]

Square count is a new algorithm