[galaxy] deal with simulation without epoch field

galaxy.py

@@ -133,15 +133,19 @@ def ring_analysis(pp, r=4, dr=0.5, dphi=0.125, z=0, dz=0.5, do_mean=True):
 
 def get_time_from_relax(pp):
     pp.load_parts()
+    try:
         epoch = pp.parts["epoch"].copy()
         epoch *= pp.info["unit_time"].express(U.Myr)
         trelax = np.min(epoch[epoch > 0])
         tfromrelax = np.max(epoch - trelax)
+    except KeyError:
+        tfromrelax = 0.0
     return tfromrelax
 
 
 def get_last_sfr(pp):
     pp.load_parts()
+    try:
         epoch = pp.parts["epoch"].copy()
         epoch *= pp.info["unit_time"].express(U.year)
         mass = pp.parts["mass"].copy()
@@ -150,6 +154,8 @@ def get_last_sfr(pp):
         masstot, time = np.histogram(epoch[mask], weights=mass[mask], bins=100)
         dtime = np.diff(time)
         sfr = masstot[-1] / dtime[-1]
+    except KeyError:
+        sfr = 0.0
     return sfr