Add units to vpts output

[PietrH]

During dinner yesterday we had a brief discussion, I'll try to summarise.

as_vpts() can output a tibble, the VPTS CSV standard specifies units. We can use the units package to enrich the information in these tibbles.

I think this would be a great feature and I don't think it'll be too hard to implement. units only imports Rcpp, so I don't think it'll cause the install size to become much bigger either.

[PietrH]

I wouldn't mind working on this, but I'll finish up the work on #53 first and then we'll see what the timing is like.

[bart1]

Is this something to be done for a cran release? It would mean maybe existing code breaks, I'm not sure bioRad can work with units. @peterdesmet & @PietrH what do you think about adding units? The same thing might apply to #73

[peterdesmet]

I think there are a bit too many unknowns to try to do this before the CRAN release.

[PietrH]

I'd prefer postponing this until after the CRAN release. That way we have more time to write solid tests for the units functionality.