Avoid copying binned coords in operations?

[SimonHeybrock]

Currently Scipp always makes a copy of binned coords (everything in var.bins.coords) during operations. This cannot be avoided in all cases, namely when the bin-indices are non-regular. This happens, e.g., when operating on a slice or a transpose.

However, in practice indices are regular in most cases. We could thus avoid the copy.

Advantages:

• Better performance.
• Less memory use.
• More consistent with "normal" coords, which are always shared.

Disadvantages:

• Inconsistent and potentially surprising behavior when working with slices (or other non-regular indices).

In practice the performance improvement and memory reduction can be quiet significant, in common cases we might expect a 2x memory reduction and 2x performance improvement.