Enforce InfiniteMPOHamiltonian to be square

[lkdvos]

After further consideration, for now the easiest solution is to enforce InfiniteMPOHamiltonian to be square. This addresses #214 .

[Gertian]

Fair enough.
Although this slightly lowers performance right ?

[lkdvos]

With the current implementation, it probably makes no difference.

[Gertian]

I meant towards an optimal implementation that considers this.

But in any case the main cost (which is the linear problem right ?) is probably mostly left unaltered ?

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/operators/mpohamiltonian.jl	80.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/operators/mpohamiltonian.jl	90.72% <80.00%> (+0.49%)	⬆️

... and 1 file with indirect coverage changes