Avoid hisat2 index error when BaseName.ss exist.

[yduanBioinfo]

No description provided.

[yduanBioinfo]

In process Hisat2Align:
index_base is named after the first file in Hisat2Index directory. Here is the code:
index_base = index[0].toString() - ~/(\.exon)?(\.\d)?(\.fa)?(\.gtf)?(\.ht2)?$/
However, when Hisat2Index/${fasta.baseName}.ss file to be the first file, the error "could not locate index" will occur.