Add linear survey

SimPEG/Survey.py

@@ -375,3 +375,11 @@ class BaseSurvey(object):
         self.dobs = self.dtrue+noise
         self.std = self.dobs*0 + std
         return self.dobs
+
+class LinearSurvey(BaseSurvey):
+    def projectFields(self, u):
+        return u
+    
+    @property
+    def nD(self):
+        return self.prob.G.shape[0]