Make `value` in `obs` and `return_type` in other funcs work the same

[zachary-foster]

These do very similar things, so they might as well have the same name and implementation. Maybe rename return_type to value and make it work like value currently does, which is more consistent with how other functions work. This will however remove return_type's ability to return taxa and hierarchies objects. I think the added flexibility of being able to use everything in all_names makes it worth it.

• [x] obs
• [x] subtaxa
• [x] leaves
• [x] roots
• [x] stems
• [x] supertaxa

[sckott]

This will however remove return_type's ability to return taxa and hierarchies objects

but they can still be accessed via $ ?

[zachary-foster]

The list of taxa would still be in the object, but the function would not convert its output to taxon or hierarchy objects. EG

> subtaxa(ex_taxmap)
$`1`
 [1] "3"  "7"  "12" "8"  "13" "4"  "9"  "14" "5"  "10" "15"

$`2`
[1] "6"  "11" "16" "17"

...

vs

> subtaxa(ex_taxmap, return_type = "taxa")
$`1`
$`1`$`3`
<Taxon>
  name: Felidae
  rank: family
  id: 9681
  authority: none

$`1`$`7`
<Taxon>
  name: Panthera
  rank: genus
  id: 146712
  authority: none

$`1`$`12`
<Taxon>
  name: tigris
  rank: species
  id: 9696
  authority: none

$`1`$`8`
<Taxon>
  name: Felis
  rank: genus
  id: 9682
  authority: none

$`1`$`13`
<Taxon>
  name: catus
  rank: species
  id: 9685
  authority: none

$`1`$`4`
<Taxon>
  name: Notoryctidae
  rank: family
  id: 4479
  authority: none

$`1`$`9`
<Taxon>
  name: Notoryctes
  rank: genus
  id: 4544
  authority: none

$`1`$`14`
<Taxon>
  name: typhlops
  rank: species
  id: 93036
  authority: none

$`1`$`5`
<Taxon>
  name: Hominidae
  rank: family
  id: 9604
  authority: none

$`1`$`10`
<Taxon>
  name: homo
  rank: genus
  id: 9605
  authority: none

$`1`$`15`
<Taxon>
  name: sapiens
  rank: species
  id: 9606
  authority: none

$`2`
$`2`$`6`
<Taxon>
  name: Solanaceae
  rank: family
  id: 4070
  authority: none

$`2`$`11`
<Taxon>
  name: Solanum
  rank: genus
  id: 4107
  authority: none

$`2`$`16`
<Taxon>
  name: lycopersicum
  rank: species
  id: 49274
  authority: none

. . .

vs

> ex_taxmap$taxa
$`1`
<Taxon>
  name: Mammalia
  rank: class
  id: 9681
  authority: none

$`2`
<Taxon>
  name: Plantae
  rank: kingdom
  id: 33090
  authority: none

$`3`
<Taxon>
  name: Felidae
  rank: family
  id: 9681
  authority: none

$`4`
<Taxon>
  name: Notoryctidae
  rank: family
  id: 4479
  authority: none

$`5`
<Taxon>
  name: Hominidae
  rank: family
  id: 9604
  authority: none