Fix tiledb_config_unset

[cgiachalis]

The config function tiledb_config_unset was meant to return the modified object of class tiledb_config, an S4 class with slot ptr with external pointer. Instead, it returns the external pointer. This PR fixes the issue by using the internal constructor intended for that purpose.

Added unit tests to check the expected class and that value is unset as intended; in addition polish and fixes the documentation in the same file Config.R.

Ref 42a0c17

Reprex:

library(tiledb)

cfg <- tiledb_config()

# Retrieve param
cfg["sm.group.timestamp_end"]
#> sm.group.timestamp_end 
#> "18446744073709551615"

# Set param
cfg["sm.group.timestamp_end"] <- "1000"

# Check param is set
cfg["sm.group.timestamp_end"]
#> sm.group.timestamp_end 
#>                 "1000"

# Reset param
cfg <- tiledb::tiledb_config_unset(cfg, "sm.group.timestamp_end")

# Check param is reset
cfg["sm.group.timestamp_end"]
#> Error in cfg["sm.group.timestamp_end"]: object of type 'externalptr' is not subsettable

class(cfg)
#> [1] "externalptr"

# Fix (with internal cstor)
cfg <- tiledb:::tiledb_config.from_ptr(cfg)

# Check again
cfg["sm.group.timestamp_end"] # OK
#> sm.group.timestamp_end 
#> "18446744073709551615"

[mojaveazure]

Looks good to me, thanks for taking care of this @cgiachalis!!