Add reflect in z axis option

[rlbarker]

I have added an option to plot rz slices reflected in the z axis, so that it is easier to visualise the entire cylindrical geometry. You can turn this functionality on/off my making reflect_in_z True or False, but it defaults to False. This fixes issue #11.

I have also added to one example to demonstrate this functionality. It changes the rz ring source plot from:

to:

The only problem is that the colorbar is not resized in the reflected plot, so it is larger than the image. Does anyone know how to fix this?

There may be a better way to implement this, so feedback is appreciated!

I have run the tests and they passed but there were several warnings - were these present before? I have also checked that all the examples still run.

[shimwell]

Thanks very much this is nice.

Could we add a little test that checks the x axis range doubles when the flag is set to True

perhaps something like this

# plot does not have mirro enabled
assert plot.get_xlim()[0] == 0
assert plot.get_xlim()[0] == 100

# plot has mirror enabled
assert plot.get_xlim()[0] == -100
assert plot.get_xlim()[0] == 100

[shimwell]

super, tests are passing. merged in and I shall make a release now