Adding set_smap_credentials and checking for 401 response

[mbjoseph]

This commit adds a function set_smap_credentials() that users can use to specify their NASA Earthdata credentials. It also implements checks for 401/unauthorized responses from the SMAP data server, which indicate that the credentials used are not authorized (would be expected if users enter the wrong username and/or password).

Big thanks to @marcosci and @ldecicco-USGS for suggesting this functionality. This should solve #43 and #41.

[codecov-io]

Codecov Report

Merging #46 into master will increase coverage by 1.33%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #46      +/-   ##
=========================================
+ Coverage   98.16%   99.5%   +1.33%     
=========================================
  Files           5       6       +1     
  Lines         273     401     +128     
=========================================
+ Hits          268     399     +131     
+ Misses          5       2       -3

Impacted Files	Coverage Δ
R/extract_smap.R	100% <100%> (+2.77%)	:arrow_up:
R/zzz.R	100% <100%> (ø)	:arrow_up:
R/set_smap_credentials.R	100% <100%> (ø)
R/download_smap.R	100% <0%> (ø)	:arrow_up:
R/find_smap.R	98.05% <0%> (+0.49%)	:arrow_up:

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[mbjoseph]

Well - this all works locally, but test coverage is now reduced because we are skipping tests that modify the global .Renviron file on Travis. I'm not sure whether this is because the .Renviron file does not exist on Travis, or because it exists but the user associated with the build does not have permissions for the file...