Open path integral calculations for momentum distribution. 
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`one_path`: An open-path integral calculation with one atom represented with an open path while all others are represented with a closed path. This setup can be used for rigorous estimation of the momentum distribution of a particle. 

`multi_path`: An open-path integral calculation with multiple atoms of a single atom type are represented with an open path while all others are represented with a closed path. Practically, this allows to average the momentum distribution over multiple atoms but it is not a rigorous way to estimate the momentum distribution of an atom if the kinetic energies of the different atoms are correlated.  
