Implement operator== for mdarray

[stefanbajakobsson]

It would be convenient if the operator== would be implemented for the class mdarray.

[mhoemmen]

I wouldn't object to P1684 changing to include an operator==, as long as operator== is constrained on the element type being equality comparable.

operator<=> wouldn't make so much sense. std::vector::operator<=> is defined in terms of the order of elements. It iterates from .begin() to .end() and returns operator<=> evaluated on the first non-equivalent pair of elements. mdarray deliberately does not expose an iterator range or define a 1-D iteration order.

[mhoemmen]

@crtrott @dalg24 @nmm0

[crtrott]

Agreed.