plot_utils: move pre_plot_setup and post_plot_setup to plot_utils

I need it for plot_allfreqs() which is not an object inheriting from
ThermalBase
diff --git a/cr2/plot_utils.py b/cr2/plot_utils.py
index 4b4f163..da892ef 100644
--- a/cr2/plot_utils.py
+++ b/cr2/plot_utils.py
@@ -32,3 +32,34 @@
         title = opt_title + " - " + title
 
     return title
+
+def pre_plot_setup(width=None, height=None):
+    """initialize a figure
+
+    width and height are numbers, ylim is a tuple with min and max
+    values for the y axis.  This function should be called before
+    any calls to plot()
+    """
+
+    set_plot_size(width, height)
+
+    _, ax = plt.subplots()
+
+    return ax
+
+def post_plot_setup(ax, title="", ylim=None):
+    """Set xlabel, title and ylim of the plot
+
+    This has to be called after calls to .plot()
+    """
+
+    plt.xlabel("Time")
+    if title:
+        plt.title(title)
+
+    if not ylim:
+        cur_ylim = ax.get_ylim()
+        ylim = (cur_ylim[0] - 0.1 * (cur_ylim[1] - cur_ylim[0]),
+                cur_ylim[1] + 0.1 * (cur_ylim[1] - cur_ylim[0]))
+
+    ax.set_ylim(ylim[0], ylim[1])