Simplify tensor structure and operations in the presence of IdentiyOp…

[albertomercurio]

This PR simplifies the structure of a TensorProductOperator in presence of IdentityOperator. This is particularly useful when expressing operators of the form

$$ I \otimes \dots \otimes I \otimes O_i \otimes I \otimes \dots \otimes O_j \otimes I \otimes \dots \otimes I $$

It also avoid to allocate some buffer cache arrays when the outer operator is the Identity, because the equations simplifies a lot.

[ChrisRackauckas]

Add tests?

[albertomercurio]

I was adding some other tests on the mul! function. I will open another PR then