Eurofins does not use unique vendor codes for their modifications:

https://eurofinsgenomics.com/en/products/dnarna-synthesis/mods/

For example, Biotin TEG can be used on the 5' or 3' end, but both use the code [BIOTEG].

Deal with this somehow, now that scadnano has switched to using vendor codes as the unique ID for a modification.

Perhaps we can store three separate Dicts, one for 5', 3', internal, to store modifications in the design. Alternatively, we could go back to what the web interface used to do, and prepend "5'-", "3'-", or "int-" to the front of the key.

I like the first solution better, to match how we split the storage of Modifications in Strands.