Figure handling

[MartinMohrmann]

This pull-request changes how figures are handled in the creation of GliderTools plots. Previously, the plt.tight_layout method was applied to all plots, making it impossible to use GliderTools for figures with advanced subplot layouts or manual matplotlib spacing changes in existing plots.
I changed the implicit manipulation of the Figure object for two reasons:

1. The figure object is not an argument of the plot functions, thus a change of the figure parameters is unexpected and should be avoided.
2. The creation of a figure object with other defaults than the matplotlib environments in the altered code felt unnecessary, the parameters outdated, and the code unflexible.

Changes are documented in the whatsnew.rst