Fix MPOHamiltonian constructors for adding multiple terms on the same site

[lkdvos]

This fixes an issue where operators that had repeating terms were wrongly discarded:

H = FiniteMPOHamiltonian(fill(ComplexSpace(2), 2), 1 => O1, 1 => O2)

would previously overwrite O1 with O2 instead of adding the two.

First caught in QuantumKitHub/MPSKitModels.jl#43

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/operators/mpohamiltonian.jl	89.23% <100.00%> (ø)

... and 2 files with indirect coverage changes