Visualization improvements

Addresses #148 (closed)

List of changes:

• plot_ensemble_mean() becomes plot_forecast_map(). Different forecast statistics can be computed, the options are 'mean', 'median' and 'iqr' and they can be specified through the parameter 'method' in the recipe. If more than one method is specified, Visualization() will iterate through all the methods.
• plot_most_likely_terciles() checks whether probabilities is a list or an array with a 'bin' dimension, to be able to work with the output of the Crossval_* modules.
• New recipe parameter custom_dictionary to specify the dictionary to be used from modules/Visualization/var_long_names.yml. The new function .get_variable_longname() includes a tryCatch() that attemps to find a custom name in the dictionary file. If the name is not defined, the name from the s2dv_cube is used.
• New shapefile parameter to specify the path of a shapefile to include in plots.
• Update PlotRobinson() and PlotEquiMap() to VizRobinson() and VizEquiMap(), respectively
• Add plot caption to plots drawn with VizEquiMap() ('cylindrical_equidistant' projection)
• Change file format conversion and add_logo() to be applied to each plot individually, in order to make them compatible with multiple atomic recipes running at the same time.
• Remove 'Units:' from metric plot captions.