Add element access via at() to std::mdspan

[stephanlachnit]

Closes #300

Possible implementation for ::at element access to mdspan with boundary checks. The boundary check is implemented with a for loop, as I did not see any other way to achieve this. The error message is inspired by the std::span::at error message from libstdc++ (gcc-mirror/gcc@1fa85dc).

Note that this does not implement ::at element access to mdarray.

[stigrs]

Given that SizeTypes can be signed, it should also check for indices[r] < 0.

[stephanlachnit]

Given that SizeTypes can be signed, it should also check for indices[r] < 0.

Thanks for this comment! Is this true for std::span as well? At least in libstdc++, they do not check this:

https://github.com/gcc-mirror/gcc/blob/fd7dabc116b9abc40ee6aa25bcc5d240b8cc516a/libstdc%2B%2B-v3/include/std/span#L290-L300

[nmm0]

In span, I believe it's always just std::size_t, but mdspan has the size type as a template parameter for the extent and can be signed.

[stephanlachnit]

In span, I believe it's always just std::size_t, but mdspan has the size type as a template parameter for the extent and can be signed.

Ah right, thanks for pointing this out!

Edit: updated the pull request

[mhoemmen]

SizeType doesn't need to be integral here or even less-than comparable with int. mdspan behaves as if all operations on indices happen after conversion to index_type. Thus, should we consider instead converting to index_type first, as we do below?

Suggested change

	// Check for negative indices
	if constexpr(std::is_signed_v<SizeType>) {
	if(index < 0) {
	return true;
	}
	}
	// Check for negative indices
	if constexpr (std::is_signed_v<index_type>) {
	if (static_cast<index_type>(index) < 0) {
	return true;
	}
	}