Only unpack resources if necessary

[FynnFreyer]

It would be nice if hivtrace would only unpack resource files if they're not found. This would have two advantages:

1. Packaging hivtrace in a container with a read only FS (like apptainer) becomes feasible.
2. Minor performance improvement after the first run.

The following code would need to be changed:

https://github.com/veg/hivtrace/blob/e1164ddbb2bd6d0d7d7a8163e6c47f2cf7f44995/hivtrace/hivtrace.py#L67-L70

An implementation could look like this:

def gunzip_file(zip_file, out_file):
    if not os.path.isfile(out_file):
        with gzip.open(zip_file, 'rb') as f_in:
            with open(out_file, 'wb') as f_out:
                f_out.writelines(f_in)

Or, using a compound with statement:

def gunzip_file(zip_file, out_file):
    if not os.path.isfile(out_file):
        with gzip.open(zip_file, 'rb') as f_in, open(out_file, 'wb') as f_out:
            f_out.writelines(f_in)