Extract tests from code in GN1

Issue:
https://github.com/genenetwork/gn-gemtext-threads/blob/main/topics/gn1-migration-to-gn2/clustering.gmi

* gn3/computations/slink.py: Add dummy `nearest' function
* tests/unit/computations/test_slink.py: Add some tests

  This commit adds tests to try and reproduce the working of the `nearest'
  function in:
  https://github.com/genenetwork/genenetwork1/blob/master/web/webqtl/heatmap/slink.py

  This commit might not yet have extracted all the expected behaviour of the
  `nearest' function, and therefore, there is a possibility of a later commit
  augmenting the work in this commit.

1 files changed, 8 insertions, 0 deletions

diff --git a/gn3/computations/slink.py b/gn3/computations/slink.py
new file mode 100644
index 0000000..bec8597
--- /dev/null
+++ b/gn3/computations/slink.py
@@ -0,0 +1,8 @@
+class LengthError(BaseException):
+    pass
+
+class MirrorError(BaseException):
+    pass
+
+def nearest(lists, i, j):
+    return None