Compatibility with Numpy 2.0

At some point (soon) we should upgrade Ketos to be compatible with Numpy 2.0.

For example, running the test suite of the utils.py module with numpy==2.0.2, already produces three errors:

$ pytest ketos/tests/test_utils.py
============================================================================= short test summary info =============================================================================
FAILED ketos/tests/test_utils.py::test_tostring - ValueError: setting an array element with a sequence. The requested array has an inhomogeneous shape after 1 dimensions. The detected shape was (2,) + inhomogeneous part.
FAILED ketos/tests/test_utils.py::test_floor_round_up - ValueError: Unable to avoid copy while creating an array as requested.
FAILED ketos/tests/test_utils.py::test_ceil_round_down - ValueError: Unable to avoid copy while creating an array as requested.
========================================================================== 3 failed, 13 passed in 0.28s ===========================================================================