Get shortest distance from two lists/tuples of member coordinates

* gn3/computations/slink.py: add code to ensure new test passes
* tests/unit/computations/test_slink.py: new test

  This one is a little weird: from
  https://github.com/genenetwork/genenetwork1/blob/master/web/webqtl/heatmap/slink.py#L57-L63

  It gets rid of the last coordinates in both the lists of the member
  coordinates, and uses the remaining coordinates to find the shortest
  members.

  For example, given the following member coordinates:
  - i=[0,1,2] and j=[5,7,9], it uses [0,1] and [5,7]
  - i=[3,6,1] and j=[7,13], it uses [3,6] and [7]
  to find the shortest distances.

  I (fredmanglis) am not sure why it does it this way, since I'd have expected
  it to use all the coordinates, however, since at this time we need to retain
  bug-compatibility with the older code, I have done it as it is done in the
  old code.

  I also add a statement to raise an exception in the case where i and j are
  not lists of integers, or integers

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