How does preferunits work?

Dear developers,

I would like to go to a convention where MeV are the preferred unit. However, this does not work:
```
Unitful.preferunits(u"MeV")

preferunits can only be used with a unit that has a pure dimension, like 𝐋 or 𝐓 but not 𝐋/𝐓.
```
What is the correct syntax?

Best,

v.

Start with the Unitful.preferunits(u"MeV") example and the reported pure-dimension error. Determine the correct syntax or limitation for preferring MeV, then document the usage and verify that the resulting guidance addresses the question.

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