Inset axes

[anekimken]

I added an option to specify existing axes for the new plots to go in. Most of the changes are in plotter.py. When the user specifies the axes using the parameter ax=my_ax, the rawdata plot is created in those axes, and the contrast plot is created as an inset axes next to those axes. This avoid creating an entirely new set of axes, since the inset is connected to the original axes.

I did my best to maintain the spacing of the plots as best I understand them, but sometimes matplotlib spacing seems like wizardry to me, so I may have overlooked something.

I also modified one of the tests in the plotting test file and created a new test file. In my hands, the new code passes the existing tests, although it does throw a warning related to a DeprecationWarning for something that will change in python 3.8. Looks like this is only a problem in matplotlib 3.0, and it gets fixed in matplotlib 3.1.

For users who don't use this option, nothing should change. For users who do, the rawdata axes are not necessarily accessible using FigName.axes[0] because the specified axes might have a different index. The contrast axes can be accessed using ax.contrast_axes where ax is the specified axes.

[josesho]

Looks good. Will merge into the 0.2.6 dev branch.