Always include z in is_temp check

[j-fannon]

At the moment Z is not read from an OBSTABLE even though it exists. Seems to be due to the new is_temp check from 2b6d631 since harp_params$z does not have a v component. Example below:

library(harp)
library(RSQLite)
library(dplyr)
library(here)

harpIO::read_obs(
  dttm               = harpCore::seq_dttm("2025030100","2025030200"),
  file_path          = here("TMP_Z_ISSUE/vobs"),
  output_format_opts = harpIO::obstable_opts(path = here("TMP_Z_ISSUE"))
)
db <- dbConnect(SQLite(),here("TMP_Z_ISSUE/OBSTABLE_2025.sqlite"))
df <- tbl(db,"TEMP") %>% as_tibble()
dbDisconnect(db)
obs <- read_point_obs(
  seq_dttm("2025030100","2025030200"),
  parameter = "Z",
  file_path = here("TMP_Z_ISSUE"),
  vertical_coordinate = "pressure"
)

gives

Warning: 'Z' not found

and

> head(df)
# A tibble: 6 × 13
  valid_dttm   SID   lat   lon  elev     p     Z     T    RH     D     S         Q    Td
       <int> <int> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>     <dbl> <dbl>
1 1740787200  1415  58.9  5.67    37  1000   229  277.  87.1   204     3  0.00449   276.
2 1740787200  1415  58.9  5.67    37   925   859  273.  75.8   254     6  0.00310   269.
3 1740787200  1415  58.9  5.67    37   850  1531  270.  49.6   269     6  0.00172   261.
4 1740787200  1415  58.9  5.67    37   700  3049  265.  17.4   274     7  0.000507  244.
5 1740787200 10618  49.7  7.33   377  1000   222   NA   NA      NA    NA NA          NA 
6 1740787200 10618  49.7  7.33   377   925   849  272.  96.3    59     8  0.00371   272.

I'm sure there is a better way than this hardcoding.