Correct seq_len for feature dict

[ivkireev86]

rec = {
    'mcc': [0, 1, 2, 3],
    'target_distribution': [0.1, 0.2, 0.4, 0.1, 0.1, 0.0],
}

How to get correct seq_len.
true len: 4
possible length: 4, 6
'target_distribution' is incorrect field to get length, this is not a sequence, this is an array

[ivkireev86]

Solved in #43