Bugfix for TDEM magnetic dipole sources (simpeg#1572)

#### Summary
Bugfix so that the initial fields produced for `db/dt`, `dh/dt`, from
the B field or H field simulations are zero. There was previously a bug
where we set the electric source term to `Zero()` when infact it should
have been evaluated. This would only effect `db/dt` (or `dh/dt`) that
were measured before the first time-step, but it also impacted
visualizations of the fields.

To catch this issue, I have extended our cross-check tests for tdem
simulation to also include an evaluation of the data at t=0. Running
this test on the current main version of SimPEG will fail.

simpeg/electromagnetics/time_domain/sources.py

@@ -1428,11 +1428,7 @@ def s_e(self, simulation, time):
                 self.waveform.has_initial_fields is True
                 and time < simulation.time_steps[1]
             ):
-                if simulation._fieldType == "b":
-                    return Zero()
-                elif simulation._fieldType == "e":
-                    # Compute s_e from vector potential
-                    return C.T * (MfMui * b)
+                return C.T * (MfMui * b)
             else:
                 return C.T * (MfMui * b) * self.waveform.eval(time)
 
@@ -1443,11 +1439,7 @@ def s_e(self, simulation, time):
                 self.waveform.has_initial_fields is True
                 and time < simulation.time_steps[1]
             ):
-                if simulation._fieldType == "h":
-                    return Zero()
-                elif simulation._fieldType == "j":
-                    # Compute s_e from vector potential
-                    return C * h
+                return C * h
             else:
                 return C * h * self.waveform.eval(time)
 

tests/em/tdem/test_TDEM_crosscheck.py

@@ -35,7 +35,7 @@ def setUp_TDEM(
     )
     mapping = maps.ExpMap(mesh) * maps.SurjectVertical1D(mesh) * activeMap
 
-    rxtimes = np.logspace(-4, -3, 20)
+    rxtimes = np.hstack([np.r_[0], np.logspace(-4, -3, 20)])
 
     if waveform.upper() == "RAW":
         t0 = 0.006