implement index class set operations methods

[wkdavis]

Signed-off-by: wkdavis william.davis@worthingtonindustries.com

[wkdavis]

@earowang If you have a moment to take a look at this, I'm not sure why namespace WARNINGs are being generated. I am not able to reproduce them locally.

> sessionInfo()
R version 3.6.1 (2019-07-05)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS  10.16

Matrix products: default
LAPACK: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] tsibble_0.9.4  testthat_3.0.0

loaded via a namespace (and not attached):
 [1] tidyselect_1.1.0  xfun_0.18         remotes_2.2.0     purrr_0.3.4       rematch2_2.1.2    diffobj_0.3.2     rcmdcheck_1.3.3   vctrs_0.3.4       generics_0.1.0    usethis_1.6.3    
[11] rlang_0.4.8       pkgbuild_1.1.0    pillar_1.4.6      glue_1.4.2        withr_2.3.0       waldo_0.2.2       xopen_1.0.0       sessioninfo_1.1.1 lifecycle_0.2.0   stringr_1.4.0    
[21] commonmark_1.7    anytime_0.3.9     devtools_2.3.2    memoise_1.1.0     knitr_1.30        callr_3.5.1       ps_1.4.0          fansi_0.4.1       Rcpp_1.0.5        backports_1.1.10 
[31] desc_1.2.0        pkgload_1.1.0     fs_1.5.0          digest_0.6.27     stringi_1.5.3     processx_3.4.4    dplyr_1.0.2       rprojroot_1.3-2   cli_2.1.0         tools_3.6.1      
[41] magrittr_1.5      tibble_3.0.4      crayon_1.3.4      pkgconfig_2.0.3   ellipsis_0.3.1    xml2_1.3.2        prettyunits_1.1.1 lubridate_1.7.9   assertthat_0.2.1  roxygen2_7.1.1   
[51] rstudioapi_0.11   R6_2.4.1          compiler_3.6.1   

[wkdavis]

@earowang

Can you incorporate some my reviews, to use {vctrs} more? Can you add a couple of examples in each help page and add yourself as a contributor?

I expanded the use of {vctrs} and added myself as a contributor in DESCRIPTION. As far as examples, there is no separate documentation for these methods because I felt that the generics (from {generics}) are sufficiently documented and include examples. However, if you'd like I can add documentation specific to the year* methods that includes examples. Thanks!

[earowang]

Thank you.