fix velvet bug in parsing mixed interleaved/separate libraries

[levinas]

Velvet has a bug in parsing args which requires -interleaved to be specified explicitly in some cases.

This doesn't work:

velveth tmp 29 -shortPaired2 -fastq -separate R2.fq R1.fq -shortPaired -fastq int.pe.fastq

This works:

velveth tmp 29 -shortPaired2 -fastq -separate R2.fq R1.fq -shortPaired -fastq -interleaved int.pe.fastq

Solution: make -interleaved explicit

[levinas]

1296a9a92f0ec131b0c1a01667f9a4c9dd9a847e