Make sure taxmap functions handle lists and vectors correctly

[zachary-foster]

Data frames in taxmap$data are pretty well tested, but not lists and vectors. Lists and vectors should be even easier than tables and the functions have been desinged with this in mind, but they have not been tested much with lists and vectors.

• [x] filter_taxa
• [x] filter_obs
• [x] select_obs
• [x] obs
• [x] mutate_obs
• [x] transmute_obs
• [x] arrange_obs
• [x] sample_n_obs
• [x] sample_frac_obs
• [x] sample_n_taxa
• [x] sample_frac_taxa
• [x] tests
• [x] man examples
• [x] mention in vignette

[zachary-foster]

What do you think about how the names of lists and vectors are turned to NA when all associated taxa are filtered out, but observations are retained with taxonless = TRUE?

Also, related to issue #38, what do you think about changing the taxonless option name to remove_obs?

> (x = filter_taxa(ex_taxmap, taxon_names == "Plantae", subtaxa = TRUE, taxonless = TRUE))

<Taxmap>
  5 taxa: 2. Plantae, 6. Solanaceae, 11. Solanum, 16. lycopersicum, 17. tuberosum
  5 edges: NA->2, 2->6, 6->11, 11->16, 11->17
  3 data sets:
    info:
      # A tibble: 6 × 4
          name n_legs dangerous taxon_id
        <fctr>  <dbl>     <lgl>    <chr>
      1  tiger      4      TRUE     <NA>
      2    cat      4     FALSE     <NA>
      3   mole      4     FALSE     <NA>
      # ... with 3 more rows
    phylopic_ids:  e148eabb-f138-43c6-b1e4-5cda2180485a ... b6400f39-345a-4711-ab4f-92fd4e22cb1a, 63604565-0406-460b-8cb8-1abe954b3f3a
    foods: a list with 6 items
  1 functions:
 reaction

> x$data

$info
# A tibble: 6 × 4
    name n_legs dangerous taxon_id
  <fctr>  <dbl>     <lgl>    <chr>
1  tiger      4      TRUE     <NA>
2    cat      4     FALSE     <NA>
3   mole      4     FALSE     <NA>
4  human      2      TRUE     <NA>
5 tomato      0     FALSE       16
6 potato      0     FALSE       17

$phylopic_ids
                                  <NA>                                   <NA>                                   <NA> 
"e148eabb-f138-43c6-b1e4-5cda2180485a" "12899ba0-9923-4feb-a7f9-758c3c7d5e13" "11b783d5-af1c-4f4e-8ab5-a51470652b47" 
                                  <NA>                                     16                                     17 
"9fae30cd-fb59-4a81-a39c-e1826a35f612" "b6400f39-345a-4711-ab4f-92fd4e22cb1a" "63604565-0406-460b-8cb8-1abe954b3f3a" 

$foods
$foods$<NA>
[1] "mammals" "birds"  

$foods$<NA>
[1] "cat food" "mice"    

$foods$<NA>
[1] "insects"

$foods$<NA>
[1] "Most things, but especially anything rare or expensive"

$foods$`16`
[1] "light" "dirt" 

$foods$`17`
[1] "light" "dirt"