Fix velox for CRAN

[Jean-Romain]

This PR provides a ready to submit fix for CRAN. In summary

• Compile in C++14
• Update DESCRIPTION
• Clean stuff by updating .gitignore and .Rbuildignore
• Update doc with roxygen 7

Tested on Linux with R4.1 using gcc and clang. 0 errors, note or warning. You should also run additional tests that I can't do because it is your email that is associated to the package.

rhub::chek_for_cran()
devtools::check_win_devel()
devtools::check_win_release()