Issues related to `axom::IndexType`

We have several open questions, issues and PRs related to `axom::IndexType`.
This issue is meant to help track the progress in resolving them:

- [x] Change default for `IndexType` to `int64` ?
- @white238 wants to know why this is signed?
- @ltaylor16 notes that Fortran does not have an unsigned type (wrappers could do the cast, if needed)
- [x] https://github.com/LLNL/axom/issues/223 (Do we need axom type aliases for native types)
- [x] https://github.com/LLNL/axom/issues/43 (axom vs. sidre)
- [x] https://github.com/LLNL/axom/issues/194 (axom vs. sidre, again)
- [ ] https://github.com/LLNL/axom/issues/295 (more flexibility in quest In/Out)
- [x] https://github.com/LLNL/axom/pull/628 (usage in `axom::Map`)

General search: https://github.com/LLNL/axom/issues?q=IndexType

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`axom::IndexType` is controlled by the following config option:
https://github.com/LLNL/axom/blob/3d962ad30f8610c180e6d742e7c5e0cab4b6619d/src/cmake/AxomOptions.cmake#L12

It is defined here
https://github.com/LLNL/axom/blob/3d962ad30f8610c180e6d742e7c5e0cab4b6619d/src/axom/core/Types.hpp#L47-L51