Added automatic creation of .netrc if none exists to address issue #93

[avvalenx]

If no .netrc file is detected the program will ask if the user has an Earthdata login. If not it will give them the link to create one. If so It will prompt for credentials and automatically create a .netrc file in the home directory.

I changed the readme to say that if not .netrc file exists then one will be automatically created if the user has Earthdata login credentials.

I also added a note in the readme that the permissions for the .netrc file should be changed for mac and linux users to address issue #152