Gh fill

[gvanweerden]

Made the following changes:

• Got rid of whitespace in function
• Removed roxygen comments and replaced with .Rd file
• Simplified code using any()
• Added test in testthat
• Added comparison .Rmd for testing base vs data.table cartesian join

[MichaelChirico]

WDYT about designing this as gh_zoom() instead? And also allowing precision < nchar(geohashes) (easier to implement obviously)?

gh_fill() naming doesn't feel perfect to me, since nothing's being "filled", exactly. "zooming" feels closer to what we're doing.