Clean up the utils/ directory

Describe the bug

There are scripts in the satpy/utils directory that are poorly maintained. They should be either maintained, documented, and covered by tests, or they should be removed. Currently, these area:

convert_to_ninjotiff.py — doesn't work, relies on obsolete mpop package
coord2area_def.py — appears to work, but not very clean, functionality could be moved into satpy or pyresample with a script as a entry point
fetch_avhrr_calcoeffs.py — doesn't work, Python 2 only, uses urllib2 module which no longer exists

To Reproduce

$ python convert_to_ninjotiff.py ...
$ python coord2area_def.py france stere 42.0 51.5 -5.5 8.0 1.5
$ python fetch_avhrr_calcoeffs.py ...

Expected behavior

I expect any code to ship with Satpy to run.

Actual results

convert_to_ninjotiff.py:

Traceback (most recent call last):
  File "/home/gholl/checkouts/satpy/utils/convert_to_ninjotiff.py", line 36, in <module>
    from mpop.projector import get_area_def
ModuleNotFoundError: No module named 'mpop'

coord2area_def.py: works, but poor code

fetch_avhrr_calcoeffs.py:

Traceback (most recent call last):
  File "/home/gholl/checkouts/satpy/utils/fetch_avhrr_calcoeffs.py", line 19, in <module>
    import urllib2
ModuleNotFoundError: No module named 'urllib2'

Environment Info:

Satpy Version: main branch

pytroll / satpy

Clean up the utils/ directory #1725