[ENH] Implementing plot title for plot_series (#4038)

Adds an optional "title" parameter for the plot_series funcion in utils.

Changes:
The function plot_series now has one more parameter: suptitle (default value is None) to which user can give the title of the plot or figure.

sktime/utils/plotting.py

@@ -23,6 +23,7 @@ def plot_series(
     labels=None,
     markers=None,
     colors=None,
+    title=None,
     x_label=None,
     y_label=None,
     ax=None,
@@ -41,6 +42,8 @@ def plot_series(
         The length of the list has to match with the number of series.
     colors: list, default = None
         The colors to use for plotting each series. Must contain one color per series
+    title: str, default = None
+        The text to use as the figure's suptitle
     pred_interval: pd.DataFrame, default = None
         Output of `forecaster.predict_interval()`. Contains columns for lower
         and upper boundaries of confidence interval.
@@ -138,6 +141,10 @@ def format_fn(tick_val, tick_pos):
     ax.xaxis.set_major_formatter(FuncFormatter(format_fn))
     ax.xaxis.set_major_locator(MaxNLocator(integer=True))
 
+    # Set the figure's title
+    if title is not None:
+        fig.suptitle(title, size="xx-large")
+
     # Label the x and y axes
     if x_label is not None:
         ax.set_xlabel(x_label)