Customizable figure size (#222)

Currently, there is no easy way to change figure sizes when using high-level visualization functions
such as `plot_problem`.
Generally, I'd prefer controlling plotting through matplotlib's `rcParams`, but this change provides
at least some backwards-compatible means of changing figure sizes at a higher level by, e.g.
`petab.visualize.plotting.DEFAULT_FIGSIZE = (8, 4)`.

petab/visualize/plotting.py

@@ -20,6 +20,8 @@
 IdsList = List[str]
 NumList = List[int]
 
+# The default figure size
+DEFAULT_FIGSIZE = (20, 15)
 
 # also for type hints
 # TODO: split into dataplot and subplot level dicts?
@@ -262,7 +264,7 @@ class Figure:
     Contains information regarding how data should be visualized.
     """
     def __init__(self, subplots: Optional[List[Subplot]] = None,
-                 size: Tuple = (20, 15),
+                 size: Tuple = DEFAULT_FIGSIZE,
                  title: Optional[Tuple] = None):
         """
         Constructor.