Change growing result offset dtype to int64

[WenzDaniel]

What is the problem / what does the code in this PR do
Changes default dtype for offset in growing result to np.int64 removes the following warning from tests:

../strax/strax/utils.py:60
  /home/dwenz/mymodules/strax/strax/utils.py:60: DeprecationWarning: `np.int` is a deprecated alias for the builtin `int`. To silence this warning, use `int` by itself. Doing this will not modify any behavior and is safe. When replacing `np.int`, you may wish to use e.g. `np.int64` or `np.int32` to specify the precision. If you wish to review your current use, check the release note link for additional information.
  Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
    def growing_result(dtype=np.int, chunk_size=10000):

[JoranAngevaare]

Thanks Daniel!