MAINT big refactor of sklearn.cluster.bicluster

* Removed dead and useless code in the utils submodule;
* decoupled sklearn.base from the bicluster module (no more deep import);
* rewrote tests to test the API, not the implementation;
* added input validation to the get_submatrix method (but it still needs
  documentation, because I've no idea what to pass for data);
* got rid of the subdirectory when it turned out only one file was left.