changed plotting return types

[bobmyhill]

This PR modifies the plot, plot_colored_trajectory, scatter and legend functions so that they return the object just created (rather than the whole ax object or nothing). This allows the user to access the attributes of the object more easily.

For example, to add a text label to a scatter point, one could do the following:

fig, ax = plt.subplots()
tax = ternary.TernaryAxesSubplot(ax=ax, scale=100)

d = tax.scatter([[20., 30., 50.]], c='k')
x, y = d.get_offsets().data[0]
ax.text(x+2., y, 'data point')