improved file handling

elettra2test.py

 #!/usr/bin/python3
-
+import os
 import at
 import matplotlib.pylab as mp
-
-ring=at.load_lattice("../../machine/lattice/elettra2/srElettra2_9_4_beta.mat",energy=2.4e9)
+lattfile = "../../machine/lattice/elettra2/srElettra2_9_4_beta.mat"
+#lattfile = "~/esrf_visit/last.mat"
+lattfile = os.path.expanduser(lattfile)
+ring=at.load_lattice(lattfile, energy=2.4e9)
 
 refpts = range(len(ring) + 1)
 ring.radiation_off()