Issues related to `axom::IndexType`

[kennyweiss]

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

• 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)
• Consider removing the axom aliases to fixed width types  #223 (Do we need axom type aliases for native types)
• Conflicting definition of IndexType #43 (axom vs. sidre)
• Inconsistent sidre::IndexType and axom::IndexType #194 (axom vs. sidre, again)
• Enable InOutOctree to run with 64 bit indexes #295 (more flexibility in quest In/Out)
• Switch Map from int to IndexType for size #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

[rhornung67]

I definitely think we should converge to a single index type definition used throughout axom for the default case.

Do we ever think we would need to support different sized index types for different axom components; e.g., int32 for one and int64 for another? If so, we should build this in along side the other points in this issue.