Array Updates

From #627

I envision making the following changes in each PR (roughly in order)

• Extending Array to multiple dimensions - #627
• Replacing MCArray with Array (now that multiple dimensions are possible - #657
• Adding a View (ArrayView?) class that provides indexing/access to a pointer it does not own, while removing the external data feature from Array - #678
• Adding GPU support via a template parameter for the memory space of the Array/ArrayView - this needs to include a dynamic option so the memory space can be set at runtime - #695

Other (smaller?) feature requests/tasks:

• Support for alternate striding: see https://github.com/LLNL/axom/pull/627#discussion_r688185293 and https://github.com/LLNL/axom/pull/627#discussion_r685551077
• Using axom::StackArray instead of std::array for the return values (and internal storage) of shape() et al - this will require some feature additions to StackArray - #695
• Remove std::inner_product from index calculations so it can run on a GPU - #695
• Refactor axom::numerics::Matrix to inherit from axom::Array
• Move Array helper/related classes into their own header files - #684
• Update ArrayIterator to be aware of the shape of multidimensional arrays (https://github.com/LLNL/axom/pull/678#discussion_r729414721)
• Fuzzy equality comparisons for Arrays/ArrayViews of floating-point types (https://github.com/LLNL/axom/pull/678#discussion_r729413987) and also StackArray?
• Default-initialize memory, but provide an option (dummy argument to constructor maybe?) to leave memory uninitialized - #709
• Add another layer of policy classes that allow for the host/device annotations to match the MemorySpace when it's locked down - point of contact @samuelpmishLLNL
• Swapping Arrays located in different memory spaces - see https://github.com/LLNL/axom/pull/695#discussion_r742433506
• Dealing with allocator IDs that are "compatible" with a memory space but are different from the "primary" allocator for that space (e.g., pooled allocators) - see https://github.com/LLNL/axom/pull/695#discussion_r742433949 and https://github.com/LLNL/axom/pull/695#discussion_r743272000 - primarily we'd want to use this to make sure that we don't throw a spurious error when constructing ArrayViews or in similar contexts
• Possible extension to detail::getAllocatorID (FIXME needs clarification) - see https://github.com/LLNL/axom/pull/695#discussion_r743249511
• Ranges for iterating over each dimension - https://github.com/LLNL/serac/pull/596#discussion_r751448475
• const Array<T> to ArrayView<const T> conversion - #709
• Enhance checks for non-default-constructible types - see https://github.com/LLNL/axom/pull/709#discussion_r752507807
• Add initializer_list constructors to axom::Array - see #808