At the recent LBL Software Carpentry Workshop, it was pointed out that there's
an inconsistency within our documentation for how to create new figures with
subplots.

Indeed, most examples were using the old way, something like:

    fig = plt.figure()
    ax = plt.subplot(111) # or plt.add_subplot(111)

This patch changes a whole bunch of instances like the above to:

    fig, ax = plt.subplots()

We should strive to have a minimal amount of constants in our code,
especially unusual ones like `111`, which only make sense to Matlab
refugees.

I have left unchanged examples which were using axes keywords passed to
subplot() and add_subplot(), since those end up transforming things like:

    figure()
    subplot(111, axisbg='w')

to

    plt.subplots(subplot_kw=dict(axisbg='w'))

which isn't necessarily better.

I also did not touch most of the user_interfaces examples, since those did not
involve using plt, but instead explicitly imported Figure, and used the OO
approach on Figure instances.

Also updated instaces where the old "import pylab as p" convention was used to
use our standard "import matplotlib.pyplot as plt"

I have also updated some, but not all uses of subplot(121) etc, but I'm a bit
exhausted after doing all of these.