Only use integer indices in archives

[btjanaka]

Description

Using a single integer for archive indices makes it easier to reason about the indices in ArchiveBase.

TODO

• Switch to integer index in GridArchive
• Switch to integer index in SlidingBoundariesArchive
• Simplify ArchiveBase to always use integer indices
• Switch _occupied_indices to a fixed-size numpy array (with dimension storage_dim) and remove _occupied_indices_cols
• Only return single index column in as_pandas
• Simplify archives to handle the new simplified ArchiveBase
• Add and test methods for raveling and unraveling indices in GridArchive and SlidingBoundariesArchive -> Added but will test and document in followup PR
• Fix visualization tools
• Fix tests
• Fix ArchiveDataFrame
• Settle on data type for indices (likely np.int64 since that is what numpy converts int to) -> just int since this depends on the system
• Clean up TODOs
• Update documentation

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