[galaxy] load only needed data

galaxy.py

@@ -6,8 +6,8 @@ from plotter import U
 
 def get_gas_dm_stars(pp):
     # Load  arrays
-    pp.load_parts()
-    pp.load_cells()
+    pp.load_parts(keys=["pos", "vel", "mass", "epoch"])
+    pp.load_cells(keys=["pos", "vel", "dx", "rho"])
     cells = pp.cells
     parts = pp.parts
 
@@ -81,9 +81,7 @@ def get_sfr(pp, stars):
         mass = stars["mass"].copy()
         mass *= pp.info["unit_mass"].express(U.Msun)
         mask = epoch > 0
-        masstot, time = np.histogram(
-            epoch[mask], weights=mass[mask], bins=200
-        )
+        masstot, time = np.histogram(epoch[mask], weights=mass[mask], bins=200)
         dtime = np.diff(time)
         sfr = masstot[-1] / dtime[-1]
     except KeyError: