added def of k to utils and have getSourceDeriv take fields and vector

simpegEM/Utils/EMUtils.py

@@ -1,5 +1,10 @@
 import numpy as np
+from scipy.constants import mu_0, epsilon_0
 
 def omega(freq):
-    """Change frequency to angular frequency, omega"""
-    return 2.*np.pi*freq
+    """Angular frequency, omega"""
+    return 2.*np.pi*freq
+
+def k(freq, sigma, mu=mu_0, eps=epsilon_0):
+	w = omega(freq)
+	return np.sqrt(mu * eps * w**2 - 1j * w* mu * sigma)