demo_zbigarray: Switch to dbopen() for opening database

And this way, because dbopen() supports opening various kind of
databases (see previous commit) we can now specify type of database on
command line, e.g.

    /path/to/db
    neo://db@master
    zeo://host:port

/cc @jm

demo/demo_zbigarray.py

@@ -11,8 +11,7 @@ TODO text
 from __future__ import print_function
 
 from wendelin.bigarray.array_zodb import ZBigArray
-from ZODB.FileStorage import FileStorage
-from ZODB import DB
+from wendelin.lib.zodb import dbopen, dbclose
 import transaction
 
 from numpy import float64, dtype, cumsum, sin
@@ -63,14 +62,14 @@ def gen(signalv):
 
 
 def usage():
-    print("Usage: %s (gen|read) <db>" % sys.argv[0], file=sys.stderr)
+    print("Usage: %s (gen|read) <dburi>" % sys.argv[0], file=sys.stderr)
     sys.exit(1)
 
 
 def main():
     try:
         act = sys.argv[1]
-        dbpath = sys.argv[2]
+        dburi = sys.argv[2]
     except IndexError:
         usage()
 
@@ -80,10 +79,7 @@ def main():
     ram_nbytes = psutil.virtual_memory().total
     print('I: RAM: %.2fGB' % (float(ram_nbytes) / GB))
 
-    stor = FileStorage(dbpath)
-    db   = DB(stor)
-    conn = db.open()
-    root = conn.root()
+    root = dbopen(dburi)
 
     if act == 'gen':
         sig_dtype = dtype(float64)
@@ -106,9 +102,7 @@ def main():
     m = p.memory_info()
     print('VIRT: %i MB\tRSS: %iMB' % (m.vms//MB, m.rss//MB))
 
-    conn.close()
-    db.close()
-    stor.close()
+    dbclose(root)
 
 
 if __name__ == '__main__':