Module cealign

generate all possible ways of matching elements of A with elements of
B, using as many members of the smaller set as possible.  For example,
if A={a,b} and B={x,y,z}, the possible matches are:
    ax-by ax-bz ay-bx ay-bz az-bx az-by

Use the CEAlign algorithm to find a good alignment of the atoms in target onto the atoms in ref.

If ref or target comprise multiple disconnected chains, all possible matches are considered.