Change in type promotion. Fixes to _signal.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/_signal.py:

tests/test_record.py::TestRecord::test_1a
  /Users/tompollard/projects/wfdb-python/wfdb/io/_signal.py:2374: UserWarning: result dtype changed due to the removal of value-based promotion from NumPy. Changed from int32 to int16.
    d_signal[d_signal < 0] = d_signal[d_signal < 0] + 65536

The changes in this pull request address these issues by explicitly casting the type. I also make a couple of minor modifications for efficiency (switching to inplace addition). I plan to follow up with several additional fixes to other modules.

[tompollard]

Thanks Benjamin