Work around the regression introduced in matplotlib 1.3.1

[mstimberg]

Does what I proposed in #245. Emits the following warning (will be only printed once) when the user tries to plot a quantity array (or calls ar.ravel() directly):

As a workaround for a bug in matplotlib 1.3.1, calling "ravel()" on a quantity will return unit-less values. If you get this warning during plotting, consider removing the units before plotting, e.g. by dividing by the unit. If you are explicitly calling "ravel()", consider using "flatten()" instead. The "ravel()" function will go back to returning a quantity after the matplotlib 1.4 release.

[thesamovar]

How about making ravel() only return a unitless version if the user has matplotlib 1.3.1 installed?

[mstimberg]

How about making ravel() only return a unitless version if the user has matplotlib 1.3.1 installed?

Good idea, I'd probably raise a warning nevertheless -- in principle a user could be calling ravel directly. And teaching them about the division by units (which also makes plotting faster) is always good :)

[thesamovar]

Oh yes, I'm happy with the warning to stay.

Also, the Travis build seems to be failing.

I'll review this one later today too.

[mstimberg]

um right, I didn't update the tests to accept a unit-less result from ravel, that's why travis is failing

[mstimberg]

I made the change conditional on matplotlib 1.3.1 and corrected the tests, test suite should pass now.

[thesamovar]

Good timing, was just about to check this. :)

[thesamovar]

Tests pass here, happy to merge if Travis build passes too.

[thesamovar]

OK they pass - merging now.