BUG Fix leftover references to ETA_STR

[spencerkclark]

Some references to ETA_STR were unintentionally not converted to internal_names.ETA_STR as part of #90. This short PR fixes this bug.

[spencerahill]

Good catch. Thanks!

[spencerahill]

This is small enough that we can skip it, but let's remember moving forward to add What's New entries for all new bug fixes and features.

[spencerkclark]

@spencerahill a quick and dirty unit test for this might be to include some three-dimensional test data in our test/data/netcdf/*.nc files, and compute a vertically integrated quantity (say column integrated water vapor for example) in test_calc_basic.py. It's not obvious to me how to pin this down to a test of a single method in calc.py.

[spencerkclark]

I fully agree that we need to have at least some Travis coverage here, and not rely on us simply running things ourselves every time we update the code. @spencerahill what are your thoughts? Does my suggestion above seem reasonable?

[spencerahill]

@spencerkclark that seems like a good approach, although make sure that it's only a single timestep for package size reasons.

It's not obvious to me how to pin this down to a test of a single method in calc.py.

That's what I was thinking also.

[spencerahill]

I opened #129 to track this further