@show for sparse_MPO does not work.

Whilst trying out the imaginary time evolution MPO I found out that the @show function for sparse_MPO doesn't work. A MWE is : 
```julia
using MPSKitModels
Ham = MPSKitModels.transverse_field_ising();
expham = make_time_mpo(Ham,im*dt,WII()); 
@show expham
```
This doesn't pose any real issues but I thought it would be good to be aware of this.