Changes for R-package caTools

Version 0.1.18.0 (2020-01-15)
 - maintainer change
 - fix of CRAN test issues (C routine registration, 1, 2)
 - update of abandoned references

 - 1, help found at https://stackoverflow.com/questions/42313373/r-cmd-check-note-found-no-calls-to-r-registerroutines-r-usedynamicsymbols
 - 2, help found at https://stackoverflow.com/questions/50787591/registering-routines-for-r-package-submission-to-cran

Version 0.1.18.1 (2021-01-08)
 - sample.split, documentation updated to explain subset assumptions
 - runmad, NA/NaN handling corrected. Thanks to suggestion by W.C. May