download should use CHECKSUMS

[GavinHuttley]

The ensembldb3.download.Download class should check that all downloaded files match the value in Ensembl's provided CHECKSUMS file.

Add a function based on the following code that is called by Download.

def get_invalid_files(path):
    """return files with invalid checksums"""
    checkpath = (path / "CHECKSUMS")
    failed = []
    with open(checkpath) as infile:
        for record in infile.readlines():
            record = record.split()
            filepath = path / record[-1]
            if not filepath.exists():
                continue

            result = exec_command(f"sum {filepath}")
            result = result.split()
            if result != record[:2]:
                failed.append(filepath)
    return failed