Build warnings

[cbrnr]

Building the conda-forge package in https://github.com/conda-forge/sleepecg-feedstock/pull/19#issuecomment-1506630898 revealed two warnings:

• [ ] There are two setuptools-related warnings for sleepecg.classifiers and sleepecg.test. These directories have been added automatically for now, but in the future this might not be the case. We could either add them manually in setup.cfg or pyproject.toml (https://setuptools.pypa.io/en/latest/userguide/package_discovery.html), or add empty __init__.py files in these directories.
• [ ] There are compiler warnings which we should probably fix:

    sleepecg/_heartbeat_detection.c: In function '_thresholding':
  sleepecg/_heartbeat_detection.c:475:45: warning: 'RR_high_limit' may be used uninitialized in this function [-Wmaybe-uninitialized]
    475 |                     if (RR_low_limit < RR_n && RR_n < RR_high_limit)
        |                         ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
  sleepecg/_heartbeat_detection.c:475:24: warning: 'RR_low_limit' may be used uninitialized in this function [-Wmaybe-uninitialized]
    475 |                     if (RR_low_limit < RR_n && RR_n < RR_high_limit)
        |                        ^
  sleepecg/_heartbeat_detection.c:330:33: warning: 'PEAKI' may be used uninitialized in this function [-Wmaybe-uninitialized]
    330 |                     SPKI = 0.25 * PEAKI + 0.75 * SPKI;
        |                            ~~~~~^~~~~~~