Warn if the index is older than the fasta/fastq file.

One may forget updating the index file and wonder why they cannot extract (find) the specific sequence that really exist in the fasta/fastq file. The index file should be constructed based on the fasta file and thus would have newer timestamp than the fasta file under normal condition. Giving a warning message when the index file is older than the fasta file may help them to understand the situation.  The order of time stamp may be disturbed when copied without -p option, but I feel the warning outweigh the silence in such cases.