remove unused matplotlib imports
Comments out unused matplotlib imports in network.py that result in problems when users try to use skrf without matplotlib installed (see
@@ -176,9 +176,9 @@ from .constants import ZERO, K_BOLTZMANN, T0
from .constants import S_DEFINITIONS, S_DEF_DEFAULT
-from matplotlib import cm
-import matplotlib.pyplot as plt
-import matplotlib.tri as tri
+#from matplotlib import cm
+#import matplotlib.pyplot as plt
+#import matplotlib.tri as tri
#from scipy.interpolate import interp1d
