wrap the C function for speed

[japhir]

Stephen Meyers suggested a simple way of using

system2("./snvec.x", "-1000", "1", "0", "/path/to/cachedir", "full-ZB18a.dat")

He did warn me that this will not pass CRAN checks however.

This is awesome!

After quickly asking on #R, people there agreed that first doing it this way and just publishing it on github would be a nice start, then later on I could rewrite the C source code so that the R functions can use .C() directly.

I've built this locally and it works great! Will see when/if I can put it in a separate branch on github.

I've added a valid (new default) output = "source" option. If it cannot find the executable it will fall back to the slow implementation. Had to rewrite some of the tests but was pretty smooth!

Following what aitap told me on #R and https://cran.r-project.org/doc/manuals/R-exts.html#Package-subdirectories

• Added the raw c source code to the src/ directory
• Added src/Makefile that builds it to snvec.x, makes it executable, then moves it to inst/
• Added src/install.libs.R that autmatically should copy the prebuilt files to the correct folder
• use system.file() to find where the newly compiled snvec.x is located
• Change snvec.R so it calls that executable with system2().

Also tried if Windows will play nice with it via devtools::check_win_devel(). It doesn't seem to source the Makefile.

Up next:

• Makefile shouldn't have to move the file around, because that changes depending on the OS