Make ArrayStore data methods more flexible

[btjanaka]

Description

This PR modifies ArrayStore as follows:

• Introduces a tuple return type to the retrieve and as_dict methods
• Renames as_dict to data()
• Removes as_pandas() in favor of data(return_type="pandas") and retrieve(return_type="pandas")
• Moves index to the end of return types where order matters, i.e., index is now the last column in pandas dataframes, and the last entry in tuples

TODO

Questions

Status

• I have read the guidelines in
  CONTRIBUTING.md
• I have formatted my code using yapf
• I have tested my code by running pytest
• I have linted my code with pylint
• I have added a one-line description of my change to the changelog in
  HISTORY.md
• This PR is ready to go