Replace Array external buffer functionality with ArrayView

[joshessman-llnl]

Summary

• This PR is a (refactoring, bugfix, feature, something else)
• It does the following (modify list as needed):
  • Removes the external buffer constructors from axom::Array
  • Adds an ArrayView class that wraps a pointer with indexing functionality

Possible follow-ups:

• Supporting "incomplete" indexing that returns an ArrayView - e.g.:

Array<int, 2> arr(3, 3); // 2D array aka matrix
ArrayView<int, 1> first_row = arr(0); // 1D array

[kennyweiss]

This is coming along really nicely @joshessman-llnl

Apologies if I jumped the gun a bit on this review, but I wanted to add some high-level design thoughts while you're working on this

[kennyweiss]

Is ArrayBase::swap() empty intentionally, or just not yet implemented?

[joshessman-llnl]

This is ArrayBase<T, 1> which has no member variables - in the general case though (implemented above) ArrayBase has m_dims and m_strides member variables that would need to be/are swapped.

[kennyweiss]

Thanks @joshessman-llnl !