ENH: Added tutorial on writing files.

[rstoneback]

Created in support of pysat/pysat#983 . Provided an interactive way to test files as I was writing code plus provides users the opportunity to test out file writing themselves.

To be added:

• Add in commented areas a setup geared towards making official ISTP/SPDF files (Going to make separate tutorial for this)

[JonathonMSmith]

Getting this error in the 8th code block:
PermissionError: [Errno 13] Permission denied: b'pysatDemo/pysat/testing/demo_test_file_2009001.nc'

If i add the "pysat/testing" directory then this example works really well. I installed the "load_meta_proc" branch manually in the session so I think I should be up to date for this tutorial.

[rstoneback]

Thanks for the bug report. I've pushed a fix to pysat/pysat#983

[rstoneback]

Thanks @JonathonMSmith! I will wait until the next pysat release to merge this.