Add DART compatible observation writer

[djhoese]

Is your feature request related to a problem? Please describe.
I saw a presentation by Jeffrey Anderson about the DART project he works on. If SatPy could act as a converter from satellite formats to something that DART could read as observations then the high bar of assimilating satellite/remote sensing data in to forecast models becomes very low (at least for DART users).

http://www.image.ucar.edu/DAReS/DART/index.php#What_is_DART

I've sent an email to Jeffrey which he said he'll look at when he is back from travel. I'm just making this issue as a reminder.

Describe the solution you'd like
Add a writer for what seems like a DART-specific format:

http://www.image.ucar.edu/DAReS/DART/DART2_Observations.php#obs_real

Describe any changes to existing user workflow
None.

Additional python or other dependencies
For testing this would probably require an actual DART environment/installation which may not be the easiest thing to do compared to a user/developer's traditional python environment...unless they could get it on conda-forge 😉

Describe any changes required to the build process
None.

Describe alternatives you've considered
Not doing it?

Additional context
Jeffrey Anderson was at SSEC/AOS at the University of Wisconsin to present on DART on September 24th, 2018. Afterward I talked with him about possibly using SatPy as a converter to get more observations in to SatPy. He seemed very interested. I am now just waiting for an email reply from him.

[djhoese]

Last I heard Jeffrey had forwarded my response on to someone on his team. I haven't heard anything from them (judging by my inbox). He mentioned using their legacy "obs_seq" format. This being a legacy format and not something people are asking for, I'm not sure this is worth he time for our team. If someone is familiar with these two projects (Satpy and DART), let me know and we can work on figuring this out. For now though, I'm going to close this.