add [[ accessor (closes #44)

[eddelbuettel]

@lsilvest when you have a moment to glance at it -- this just fills in a gap we never needed filled but #44 tells us that dplyr was unhappy about this

[codecov]

Codecov Report

Merging #45 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #45   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          92     93    +1     
=====================================
+ Hits           92     93    +1

Impacted Files	Coverage Δ
R/nanotime.R	100% <100%> (ø)	⬆️

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[lsilvest]

It looks good to me. I verified it correctly drops names and also that it errors out with the correct error message when more than one element is selected.

[eddelbuettel]

Beauty of S4 :)

I'll merge and wrap this up. Should be on CRAN within minutes (once I wrap up the usual tests here, on winbuilder and rhub) as we have a clean sheet here.

[randomgambit]

amazing job guys! I will run more tests on my side to check everything is OK. Thanks