add function to extract points from rasters

[ThierryO]

No description provided.

[stijnvanhoey]

Thanks a lot for the contribution; being able to query radar rainfall (and other spatial products?) is a long standing open issue, see #51 (and wrapping GetRasterToPointValues is probably the most usefule of the calls enlisted in the issue).

@damianooldoni or @peterdesmet are you able to review the R-implementation? (there is some syntax I am not really up to date with to properly review)

(I am on holiday for the moment and won't be able to further check till half August, so @peterdesmet feel free to merge if all fine).

[stijnvanhoey]

Before we forget, @ThierryO please enlist the new function in the pkgdown overview file (which we manage manually to help users find the most important functions), https://github.com/ropensci/wateRinfo/blob/master/_pkgdown.yml#L23 I think this one belongs to the main/first section