Check for Earthdata credentials in netrc (adapted from Joe Kennedy/Forrest Williams) resolving isse #83
when no credentials in netrc are present when requesting data for nasadem or srtm_v3, there is a human readable error instructing user to update their ~/.netrc.
Resolves #83.
nasademorsrtm_v3, there is a human readable error instructing user to update their~/.netrc.