Different tensor types should not compare equal

[eschnett]

I find this:

julia> VectorValue(1,2,3) == SymTensorValue(1,2,3)
true

This shouldn't be the case; 3d vectors are different from 2d tensors...

[eschnett]

I think that

(==)(a::MultiValue,b::MultiValue) = a.data == b.data

should instead be

(==)(a::T,b::T) where {T <: MultiValue} = a.data == b.data

together with appropriate promotion rules. Alternatively, the comparison function could explicitly compare the tensor types (VectorValue, TensorValue, etc) of the arguments.

[fverdugo]

Thanks for noting this @eschnett. You are right!
Can you open please a PR with the proposed changes?