Make default `value` `"taxon_indexes"`

It is currently  `NULL` , which does not return names.

```
> ex_taxmap$subtaxa(subset = is_root)
$b
 [1]  3  7 12  8 13  4  9 14  5 10 15

$c
[1]  6 11 16 17

> ex_taxmap$subtaxa(subset = is_root, value = "taxon_indexes")
$b
 d  h  m  i  n  e  j  o  f  k  p 
 3  7 12  8 13  4  9 14  5 10 15 

$c
 g  l  q  r 
 6 11 16 17
```
