Plots/add utils functions

[hoyer-a]

In preparation of #211 _prepare_plot() is added similar to pyfar.plot._utils._prepare_plot():

_prepare_plot():
return figure and ax depending on ax input and projection

[f-brinkmann]

There is a large overlap with the similar functions at pyfar but I did not really see a good way of combining them, without making the structure more more complex - so approved.