Calculate a simple consensus sequence for the block.

def get_consensus(block):
    """Calculate a simple consensus sequence for the block."""
    from collections import Counter

    # Take aligned (non-insert) chars from all rows; transpose
    columns = zip(*[[c for c in row['seq'] if not c.islower()]
                    for row in block['sequences']])
    cons_chars = [Counter(col).most_common()[0][0] for col in columns]
    cons_chars = [c if c != '-' else 'X' for c in cons_chars]
    assert len(cons_chars) == block['query_length']
    cons_sequence = {
        'index': 1,
        'id': 'consensus',
        'description': '',
        'dbxrefs': {},
        'phylum': '',
        'taxchar': '',
        'head_len': None,
        'tail_len': None,
        'head_seq': '',
        'tail_seq': '',
        'length': block['query_length'],
        'seq': ''.join(cons_chars),
    }
    return cons_sequence