Decision for 1.0: Comparison operators

[dpsanders]

Important decision about comparison operators:

• Comparison operators <, >, ==, iszero, etc. have been generally removed between intervals, except for equality checks (e.g. ==, iszero, isone), where it is allowed if at least one operand is a thin interval.

  Ideally we would want to only define these operators when both intervals are singletons, and throw an ArgumentError otherwise.

  However, we ran into compatibility issues with other libraries, e.g., SparseArrays.jl (see also the PRs Allow pointwise equality #604, Allow == if one of the operand is a thin interval #631, Ensure boolean functions error for non-thin intervals #613). We should check if it is reasonable to add SparseArrays.jl as a package extension.