text: Title: Mapper / Scikit-learn error from precomputed clustering
  Body: <!-- Instructions For Filing a Bug: https://github.com/giotto-learn/giotto-learn/blob/master/CONTRIBUTING.rst -->
  
  #### Description
  I have a point cloud of 1920 nine-dimensional points. When applying Mapper with `DBSCAN` clustering as in the Christmas Santa notebook everything works fine. When I apply my own clustering algoritm with a precomputed distance matrix I get an error. Using Kepler Mapper I make this work by setting the parameter `precomputed=True` when calling `mapper.map()`.
  
  PS! I used the color function from the Santa `.csv` file as a hack to make the code run. It worked for the basic clustering method.
  
  UPDATE: I added `point_cloud.csv` to the gist, I hope it works for reproduction.
  
  #### Steps/Code to Reproduce
  https://gist.github.com/torlarse/43604dd09a98cc3f69166659cd6ddf9e
  
  #### Expected Results
  A Mapper complex :)
  
  #### Actual Results
  Please see gist for traceback. 
  
  #### Versions
  Python 3.7.5 (tags/v3.7.5:5c02a39a0b, Oct 15 2019, 00:11:34) [MSC v.1916 64 bit (AMD64)]
  NumPy 1.17.4
  SciPy 1.3.3
  joblib 0.14.0
  Scikit-Learn 0.22
  giotto-Learn 0.1.3
  
  
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