Check that the code and ICs have the same dimensionality

Does what it says on the tin. :)

Implementation of #203 (closed) following @bvandenbroucke's request. When reading ICs we check whether the ICs have the same dimensionality as the code was compiled with. We assume d=3 by default to retain compatibility with Gadget-[234].

Branch can be removed.

examples/BigCosmoVolume/makeIC.py

@@ -133,6 +133,7 @@ grp.attrs["Time"] = 0.0
 grp.attrs["NumFilesPerSnapshot"] = 1
 grp.attrs["MassTable"] = [0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
 grp.attrs["Flag_Entropy_ICs"] = 0
+grp.attrs["Dimension"] = 3
 
 #Runtime parameters
 grp = file.create_group("/RuntimePars")