`get_pvol()` test failure on `hochficht`

[PietrH]

I came across a test failure while investigating #172 , a normal test run on main.

Failure (test-get_pvol_at.R:32:3): hochficht can be combined with other radars
Expected every element of `purrr::map_lgl(pvol_lst, inherits, "pvol")` to equal TRUE.
Differences:
`actual`:   TRUE TRUE FALSE
`expected`: TRUE TRUE TRUE