Check insertion and deletion blast files

[elcortegano]

In many instances, no deletions or insertions are present in blast files, resulting in errors reported by cat and rm. For example:

cat: test.deletion_blast: No such file or directory
rm: cannot remove 'ref_segment.fa': No such file or directory
rm: cannot remove 'temp.blast2': No such file or directory

Now, insertion and deletion blast files are only sorted (and later removed) when they exist.