WIP got rid of Units class

[micahkim23]

@spencerahill How does this look?

[spencerkclark]

@micahkim23 this looks great! Thanks for putting this together. I just have one comment within the diff for the documentation.

It would also be great if you could add a note in whats-new.rst regarding this change. As @spencerahill suggests (#50 (comment)), please update the version number for the "unreleased" heading to 0.3. I think this would probably be best noted under a new "Breaking changes" heading (since this will require some users to modify their object libraries).

[spencerkclark]

I realize you're just copying this text from the old documentation for Units objects (which is totally fine), but I think it might be good to take the opportunity to be a little more descriptive here. Maybe something along the lines of:

.. note::

    While for the sake of tracking metadata we encourage users to add a ``units`` 
    attribute to each of their :py:class:`Var` objects, these ``units`` attributes provide 
    nothing more than descriptive value.  One day we hope DataArrays 
    produced by loading or computing variables will be truly units-aware 
    (e.g. adding or subtracting two DataArrays with different units will lead to 
    an error, or multiplying two DataArrays will result in a DataArray with new units), 
    but we will leave that to upstream libraries to implement
    (see `pydata/xarray#525 <https://github.com/pydata/xarray/issues/525>`_ 
    for more discussion).

[micahkim23]

@spencerkclark thanks for the suggestion, I'll fix that.

[spencerahill]

I agree, and just add 'the' between 'for' and 'sake'.

[spencerahill]

Thanks @micahkim23. Looks good overall. I agree w/ @spencerkclark 's comments. Once those are addressed this should be good to go!

[micahkim23]

I was just wondering, since this change is supposed to be part of 0.3, should the pull request not be made on the develop branch? Will it be a problem for code from 0.3 and 0.2.1 to be mixed?

[spencerahill]

There's actually nothing that is formally binding us to v0.2.1 at this point. Just change the v0.2.1 to v0.3 within the What's New, and then whenever I do the next release I will update everything (e.g. within setup.py and on our conda-forge feedstock) to v0.3 instead of v0.2.1. Only once a release is actually performed does any code get formally tied to it.

[spencerkclark]

This looks good to me; just a couple minor nits!

[spencerkclark]

• Remove the double backticks from "string" (in other words make it ordinary text rather than a literal, because the literal name of the datatype is str)
• Add a colon before "pull"

[spencerahill]

In it goes! Thanks @micahkim23 for the heavy lifting and @spencerkclark for the review.