Add AA -> mRNA backtranslation to pVACvector

[susannasiebert]

We currently backtranslate the final pVACvector vector peptide sequence from AA -> DNA via a simple lookup table. This process should be improved to take into account all possible codons and pick the most likely one. There are some tools that do so for AA -> mRNA backtranslation:

• CodonBERT: https://pubmed.ncbi.nlm.nih.gov/38951026/, https://github.com/FPPGroup/CodonBERT
• LinearDesign: https://pubmed.ncbi.nlm.nih.gov/37130545/, https://github.com/LinearDesignSoftware/LinearDesign

We want to add AA -> mRNA backtranslation to pVACvector. The existing AA -> DNA backtranslation via codon lookup could then be replaced by using the mRNA sequence and replacing Us with Ts.