Add "ask" option to EarthdataLogin to enable non-interactive login and control credentials file path

[nuest]

I would like to run a script for downloading data. I have stored my credentials in environment variable and they are used, but MODIS always asks if I want to change them:

> MODIS::EarthdataLogin(usr = Sys.getenv("EARTHDATA_USER"), pwd = Sys.getenv("EARTHDATA_PASS"))
Earthdata credentials seem to be present, do you want to change them? (y/n) 

Would you welcome a PR adding an option ask with default TRUE to the function EarthdataLogin(..) login to disable the question?

In addition:

• An option with current default credentialsPath = path.expand(~/.nrc) would give users more control, in cases where the user home is not a suitable storage location.
• In the same style als MODISoptions(..), a save = TRUE parameter could even be added to disable creating the .netrc file.

Happy to prepare a PR if you're available for review (and maybe even a minor version release?)

[nuest]

My current workaround is to do a file.remove(path.expand("~/.netrc")) before EarthdataLogin(..).

[fdetsch]

Hey @nuest,

I'd be happy to receive and review a PR from you, the suggested changes seem perfectly reasonable. EarthdataLogin() emerged as kind of a hotfix when login-restricted data download was first introduced, which I guess leaves quite a lot of space for improvement.

[nuest]

@fdetsch Sorry for the long silence - I'm just picking up this workflow again. AFAICT this is still the case, so I can give it another go.