Provide additional aliases for S4 subsetting methods

[eddelbuettel]

This suppresses an entirely harmless but utterly annoying warning that pops up every now and then. S4 methods are somewhere between wonderful, a can of worms, and a hornets. The change is this PR votes for last option. (Underlying cause may be a bug on roxygen2 as the warning comes and goes over rebuilds. Now it should be gone with a capital G.)

No code changes.

Suppresses this beauty:

✔  checking Rd cross-references
W  checking for missing documentation entries (702ms)
   Undocumented S4 methods:
     generic '[' and siglist 'tiledb_array'
     generic '[' and siglist 'tiledb_config'
     generic '[' and siglist 'tiledb_dense'
     generic '[' and siglist 'tiledb_filter_list'
     generic '[' and siglist 'tiledb_sparse'
     generic '[<-' and siglist 'tiledb_array'
     generic '[<-' and siglist 'tiledb_config'
     generic '[<-' and siglist 'tiledb_dense'
     generic '[<-' and siglist 'tiledb_sparse'
   All user-level objects in a package (including S4 classes and methods)
   should have documentation entries.
   See chapter ‘Writing R documentation files’ in the ‘Writing R
   Extensions’ manual.
✔  checking for code/documentation mismatches (2.1s)