Move ItemCollection and derived classes to core

[nselliott]

Summary

• This PR is a refactoring
• It does the following:
  • Moves the ItemCollection and its child classes from Sidre to core, to enable them to be used without depending on the Sidre component.
  • Fixes #1397 , may be useful within Sina

Moving these header-only classes required removing their dependencies on Slic, as there were a few uses of Slic assertions and warnings. Those have been replaced with raw assert() calls and output to std::cerr, matching what is done elsewhere in Axom core.

The definitions of constants InvalidIndex and InvalidName needed to be moved from Sidre into core for usage by the collection classes.

[nselliott]

Thanks @nselliott

Please update the RELEASE_NOTES with this change. Should add some unit tests to core for basic operations on these classes?

The existing set of unit tests for these classes in the sidre test directory is still good. We could move them to the core test directory if needed.