Make R CMD build happy on Windows systems.
Merge remote-tracking branch 'upstream/master'
merge pull request; simplify vignette requirements
update output
optimize coercion from STIDF to STSDF
Merge pull request #16 from BenGraeler/master

matrix like subsetting for STS and STSDF
make tracks vignette work without https, add A3
Merge branch 'master' of github.com:edzer/spacetime
remove dependencies in tracks.Rmd
- allow for multiple selection in STS..
- respect ordered subsetting from STS..
- STF.. now allows for a matrix index, but returns STS..
- STI.. allows now for multiple selection