Disable ebirdfreq(), throws error by default and provides url as message

[sebpardo]

As discussed in #82, frequency data can no longer be downloaded from the eBird website without being logged in, therefore ebirdfreq() stopped working altogether. Fortunately, it is likely that the frequency data will be available through the API in the near future, so we are keeping the function temporarily, where it throws an error while also providing the constructed url to manually download the frequency data through their website:

ebirdfreq("hotspots", "L196159")
# http://ebird.org/ebird/barchartData?r=L196159&bmo=1&emo=12&byr=1900&eyr=2021&personal=false&fmt=tsv
# Error in ebirdfreq("hotspots", "L196159") : 
#   This function has been made temporarily defunct as you now need to be 
#    logged into the eBird website to download frequency data, but we cannot 
#    authenticate via the API to do so. This function will be reinstated
#    when the frequency data become available through the API. In the meantime, 
#    you can paste the url above in your browser to obtain the web download of 
#    the frequency data for the desired location and date. See
#    https://github.com/ropensci/rebird/issues/82 for more information.

I've also updated the documentation to explain the issue with the function and its new output, and the tests.

This way we can upload a new package version that passes all tests to CRAN, and I'll update the function as soon as the functionality becomes available in the API.

Closes #82.