Remove executables and built from source code

[mrustl]

The executables:
inst/extdata/engine/linux/vs2dh3_3.exe
inst/extdata/engine/win/vs2dh3_3.exe
inst/extdata/engine/win/vs2dt3_3.exe

should be removed and build from source code:
https://wwwbrr.cr.usgs.gov/projects/GW_Unsat/vs2di1.3/vs2dt3_3.zip
https://wwwbrr.cr.usgs.gov/projects/GW_Unsat/vs2di1.3/vs2dh3_3.zip

(more info: https://wwwbrr.cr.usgs.gov/projects/GW_Unsat/vs2di1.3/index.html)

Subsequently the R code needs to be adapted to take this into account.