Change in type promotion. Fixes to edf.py

[tompollard]

As discussed in #493, numpy v2.0 introduced changes to type promotion rules: https://numpy.org/devdocs/numpy_2_0_migration_guide.html#changes-to-numpy-data-type-promotion

Running pytest with numpy==2.0.2 and NPY_PROMOTION_STATE=weak_and_warn raises the following warning for wfdb/io/convert/edf.py:

tests/io/test_convert.py::TestEdfToWfdb::test_edf_uniform
  /Users/tompollard/projects/wfdb-python/wfdb/io/convert/edf.py:409: UserWarning: result dtype changed due to the removal of value-based promotion from NumPy. Changed from int16 to int64.
    temp_all_sigs[i].flatten() - baseline[i]

tests/io/test_convert.py::TestEdfToWfdb::test_edf_non_uniform
  /Users/tompollard/projects/wfdb-python/wfdb/io/convert/edf.py:420: UserWarning: result dtype changed due to the removal of value-based promotion from NumPy. Changed from int16 to int64.
    (temp_sig_data[start_ind:stop_ind] - baseline[i])

tests/io/test_convert.py::TestEdfToWfdb::test_edf_non_uniform
  /Users/tompollard/projects/wfdb-python/wfdb/io/convert/edf.py:414: UserWarning: result dtype changed due to the removal of value-based promotion from NumPy. Changed from int16 to int64.
    sig_data[:, i] = (temp_sig_data - baseline[i]) / adc_gain_all[i]

The changes in this pull request address these issues by explicitly casting the type.

[bemoody]

This doesn't fix any of the warnings. Think about what the code is doing and what the warning says.

What is the operation that reports a warning?

What is the type and the value of the left-hand side?

What is the type and the value of the right-hand side?

What is the type and the value of the result when it's evaluated under legacy rules?

What is the type and the value of the result when it's evaluated under weak rules?

What is the desired result and why?