Checks for file download

[eliocamp]

Checks if data is ready without downloading the entire file and only when it is, it downloads directly to disk. (Closes #39)

The result is:

library(ecmwfr)
request <- list(stream = "oper",
                levtype = "sfc",
                param = "167.128",
                dataset = "interim",
                step = "0",
                grid = "0.75/0.75",
                time = "00",
                date = "2014-07-01/to/2014-07-02",
                type = "an",
                class = "ei",
                area = "50/10/51/11",
                format = "netcdf",
                target = "tmp.nc")

wf_request(request = request)

#> Requesting data to the webapi service with username eliocampitelli@gmail.com
#> - staging data transfer at url endpoint or request id:
#>   https://api.ecmwf.int/v1/datasets/interim/requests/5d72a1f713e01cd44c6f4858
#> 
#> - timeout set to 1.0 hours
#> - polling server for a data transfer
#> Downloading file
#> |=================================================================================================| 100%
#> - file not copied and removed (path == tempdir())
#> - request purged from queue!

This uses the somewhat hacky function retrieve_header. If curl or httr decide to implement the --head argument, then it could be changed to a cleaner solution.

[khufkens]

CI failed in branch, image didn't compile nicely. Will see if it works in master.