add `bundle_vector`

[markromanmiller]

usually I do something like

df %>%
    mutate(Pos = vector3(Posx, Posy, Posz), .keep="unused")

what if I could do something like

df %>%
    bundle_vector(Pos) # or bundle_vector(Pos="Pos{d}")

and have some flexibility for other specs too?

[markromanmiller]

I came to the issues page to suggest this same thing, but with a different interface:

df %>%
    bundle(vector3 = "HeadPos", quat="HeadRot") - more terse, but less expressive.

[markromanmiller]

code has just been written in thesis/mega-sync/correlation_methods.Rmd