Fix PVG that downstream node skipped mistakenly after cleaving

[zhuchcn]

Description

This very tricky issue only appears when the 6 SNVs + indels are present together. Leaving anyone out will make the issue disappear.

Issue in graph digestion. When determining the downstream nodes for the next iteration after cleaving a bubble, the outbound node of a newly created node (by merging or cleaving) is usually skipped if the new node contains frameshifting variants. However, if the outbound node contains multiple inbound nodes, and all of them are created in the current bubble, it should still be processed and identified as a downstream node, otherwise, it will remain as uncleaved and result in potential invalid characters (e.g., *).

Closes #...

Checklist

• This PR does NOT contain PHI or germline genetic data. A repo may need to be deleted if such data is uploaded. Disclosing PHI is a major problem.
• This PR does NOT contain molecular files, compressed files, output files such as images (e.g. .png, .jpeg), .pdf, .RData, .xlsx, .doc, .ppt, or other non-plain-text files. To automatically exclude such files using a .gitignore file, see here for example.
• I have read the code review guidelines and the code review best practice on GitHub check-list.
• The name of the branch is meaningful and well formatted following the standards, using [AD_username (or 5 letters of AD if AD is too long)]-[brief_description_of_branch].
• I have added the major changes included in this pull request to the CHANGELOG.md under the next release version or unreleased, and updated the date.
• All test cases passed locally.