Fix problems with saving clusters

These changes appear to fix the problems I had with saving clusters -- along the lines of:

An exception occurred: Traceback (most recent call last): 
  File "/Users/ben/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/qtools/tasks.py", line 86, in worker_loop 
    result = getattr(task_obj, method)(*args, **kwargs) 
  File "/Users/ben/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/klustaviewa/gui/threads.py", line 39, in save 
    loader.save() 
  File "/Users/ben/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/kwiklib/dataio/hdf5loader.py", line 502, in save 
    self.clusters_table.cols.group[:] = self.cluster_info['group'] 
  File "/Users/ben/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/tables/table.py", line 3304, in __setitem__ 
    value, self.pathname) 
  File "/Users/ben/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/tables/table.py", line 2229, in modifyColumn 
    "'%s'. The error was: <%s>" % (str(column), exc)) 
ValueError: column parameter cannot be converted into a ndarray object compliant with specified column '1     1 
2     2 
3     2 
4     2 
5     2 
6     2 
7     2 
8     2 
9     2 
10    2 
11    2 
12    2 
13    2 
Name: group, dtype: int32'. The error was: <objects of type ``Series`` are not supported in this context, sorry; supported objects are: NumPy array, record or scalar; homogeneous list or tuple, integer, float, complex or string>