Add SubView class

[gberg617]

Summary

• This WIP PR adds the SubView class. This class stores a view-type object and a tuple of slices that indicate how to access the view's underlying data. The following slice types are supported:
  • range slice: Indexes a subset of the original dimension's extents. Does not reduce dimensionality.
  • fixed slice: Indexes along a fixed value of a given dimension. Reduces dimensionality.
  • no slice: Indexes according to the original view's layout along the given dimension. Does not reduce dimensionality.

A few simple examples are provided below:

    View<Index_type, Layout<2>> view(&my_data[0][0], Layout<2>(3,3));

    // "sv1 = View[1:3,:]"
    auto sv1 = SubView(view, RangeSlice{1,3}, NoSlice{});

    // "sv2 = View[1:3,1]"
    auto sv2 = SubView(view, RangeSlice{1,3}, FixedSlice{1});

Here, sv1 and sv2 are subviews of dimensions (2,3) and (2) respectively.

Future tasks:

• Add Slices whose extents are determined at compile time
• Add Slice types that use non-affine mappings (e.g. index lists)
• Add ability to collapse nested sub-layouts into a single sub-layout for slices that have affine mappings
• Add ability to distinguish between projected dimension and an empty slice (e.g. RangeSlice{2,2})

[gberg617]

This PR is still a WIP!

[gberg617]

To Do: Fix GPU tests.

[gberg617]

Perhaps remove GPU tests?

[gberg617]

I'm considering separating the SubView holding the view from the slices that are used to access the view, somewhat analogous to the approach of having layouts and Views be separate classes. Any thoughts on this?

[gberg617]

@MrBurmark Did we decide to go with std::array, or switch back to camp::array here?

[MrBurmark]

Let's use camp array, we haven't asked users to add the cuda flag that would enable this yet.

[gberg617]

OK, I added back camp::array. This means that we first need to push the following camp PR:
llnl/camp#194