DeprecationWarning: np.float is a deprecated alias for the builtin float

[csala]

Running the tests on branch v0.6.0-dev issues a huge number of DeprecationWarnings about np.float not being valid any more.

tests/unit/transformers/test_numerical.py::TestNumericalTransformer::test__fit
  .../RDT/tests/unit/transformers/test_numerical.py:30: DeprecationWarning: `np.float` is a deprecated alias for the builtin `float`. To silence this warning, use `float` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.float64` here.
  Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
    transformer = NumericalTransformer(dtype=np.float, nan='nan')

We should update the code to replace np.float with float or np.float64.