Splitting of the discussion of non-broadcasted addition from #416.

1. MIMO system + constant, looks wrong (and could potentially be confusing if 1 is interpreted as something like a UniformScaling).

ss(-ones(2,2), ones(2,2), ones(2,2), zeros(2,2)) + 1

Let's disallow this. @mfalt seems to agree that this is the way to go.

2. SISO system + constant seems reasonable and should be allowed.

ss(-ones(1,1), ones(1,1), ones(1,1), zeros(1,1)) + 1

3. Should we allow sys + I? I guess it should only be allowed for square systems sys.

There are other functions like feedback where we want similar behavior. Other functions to add to this list?