Functions for converting to and from CDAT cdms2 variables

[shoyer]

Apparently CDAT has a number of useful modules for working with weather and climate data, especially for things like computing climatologies (related: #112). There's no point in duplicating that work in xray, of course (also, climatologies may be too domain specific for xray), so we should make it possible to use both xray and CDAT interchangeably.

Unfortunately, I haven't used CDAT, so it not obvious to me what the right interface is. Also, CDAT seems to be somewhat difficult (impossible?) to install as a Python library, so it may be hard to setup automated testing.

CC @DamienIrving

[DamienIrving]

@shoyer I love this suggested enhancement. If I could use xray and CDAT interchangeably, then I'd add xray into my workflow immediately (I'd image many other people would too, as CDAT has a fairly large user base).

The first thing I'd say is that you don't need to install all of UV-CDAT to get the useful modules. Instead, people have developed cdat-lite, which strips away all the visualisation stuff associated with UV-CDAT and just leaves the core convenience functions for calculating climatologies etc (i.e. it strips away the UV bit and just leaves the CDAT).

With the emergence of conda and binstar, it's now very easy to install cdat-lite with Anaconda. This page should be all you need: https://binstar.org/ajdawson/cdat-lite

[shoyer]

OK, I just tested out that conda package and it does appear to work! That is great -- this will make it possible for me to get automated tests going (which is a prerequisite to adding new features).

I'm about to do a major release but I should be able to take a look at this shortly if you are willing to test it out. It looks like the basic interface should be pretty straightforward.

[DamienIrving]

Happy to test it out for you.

[aashish24]

👍 @DamienIrving others, let us know if you need any help! This is really awesome stuff.

[shoyer]

@aashish24 This has been in the last few xray releases -- I'd love to get some feedback on it, I don't think it's gotten much use yet (I don't use cdat).

[aashish24]

Sure..we will give it a try soon and report back. I guess what would help is some documentation and may be a blog. What you think?

[shoyer]

This is technically documented, but barely :)
http://xray.readthedocs.org/en/stable/generated/xray.DataArray.from_cdms2.html#xray.DataArray.from_cdms2
http://xray.readthedocs.org/en/stable/generated/xray.DataArray.to_cdms2.html#xray.DataArray.to_cdms2

See here for a minimal example of converting to a cdms2 variable:
https://github.com/xray/xray/blob/eed58d5cb4274f262e77277bc8fcf394a9fae77c/xray/test/test_dataarray.py#L1272