Change default colormap to emulate previous colormap.

In an attempt to preserve previous behavior as much as possible,
use matplotlib's Spectral_r colormap, which resembles the Color
Brewer diverging spectral map we were previously using from
palettable.

FlowCal/plot.py

@@ -58,7 +58,7 @@
 from matplotlib.font_manager import FontProperties
 import warnings
 
-cmap_default = plt.get_cmap(matplotlib.rcParams['image.cmap'])
+cmap_default = plt.get_cmap('Spectral_r')
 
 savefig_dpi = 250