Move TravelSignal into submodule signal

simulations/lib/__init__.py

@@ -1,3 +1,6 @@
-from .travelsignal import *
+from . import signal
 from . import location
 from .util import *
+
+
+TravelSignal = signal.TravelSignal

simulations/lib/signal/__init__.py

@@ -0,0 +1 @@
+from .travelsignal import *

simulations/lib/signal/travelsignal.py

@@ -6,6 +6,11 @@ Define the TravelSignal class.
 import numpy as np
 import scipy.interpolate as interp
 
+try:
+    from _signal import *
+except ImportError:
+    from ._signal import *
+
 class TravelSignal:
     """
     Model an arbitrary digitised signal that can be translated to another position and time.