Merge different Sparse matrices with associated geometries

[zerothi]

Describe the feature
We want to be able to merge different sparse matrices since this would allow one to:

1. create complicated Hamiltonians available for transport calculations
2. create initial DM files my stitching together different DM's

[zerothi]

This is partly done by adding add which lets one merge two sparse matrices but it will not create any couplings between them.

[zerothi]

Now we have add, append/prepend and replace which should do everything.