incorrect handling of "D" in existing CIGAR strings with mask_* functions

[ahstram]

Here's an example:

print("3M1D137M",cigar.Cigar("3M1D137M").mask_left(4).cigar)

Which will yield:

3M1D137M 5S136M

Which is incorrect, as the original "3M1D137M" string has query sequence length 140, but the left-masked query sequence length of "5S136M" is 141.