Unable to use some WFDB-compliant files

[matthewreyna]

A user reported that they were unable to load WFDB files with .mat file extensions, e.g., from the PhysioNet Challenge 2020:
https://physionet.org/content/challenge-2020/1.0.2/training/georgia/g1/#files-panel

I suspect that the find_records function is causing this problem:

ecg-image-kit/codes/ecg-image-generator/helper_functions.py

Line 30 in 27b90f5

def find_records(folder, output_dir):

There are at least three issues to fix:

1. This function hard-codes the signal filename extensions. It should load the signal filenames from the header files.
2. It associates the signal files with header files by sorting the filenames, assuming that the order of the sorted filenames provides a pairing of the header and signal files. It should load the signal filenames from the header files.
3. It uses platform-specific path separators to separate and join file paths, e.g., / vs \. It should use the built-in functions from os.path.