Adding convenience function 'from_parametrized_class' to ClassSelector

Dear all,

I tried to figure out how to specify a parameter only accepting a "Parameterized" class.
I found ClassSelector to be what I wanted. But as far as I could see I could not automatically generate a ClassSelector matching my "Parameterized" class. Hence I added the classmethod 'from_parametrized_class' to ClassSelector.

My solution was very simple. (See single commit).

Here is a demo on how to use the function:

<pre>
import param

class IonType(param.Parameterized):
    name = param.String(doc='Human readable name of ion')
    charge = param.Integer(0, doc='Charge of ion.')
    mass = param.Number(doc='Mass in atomic units.')

class Ion(param.Parameterized):
    ion_type = param.ClassSelector.from_parametrized_class(IonType)
    coord = param.XYCoordinates(doc='Coordinate of ion.')

sod = IonType(name='Sodium', charge=1, mass=22.99)

ion1 = Ion(ion_type=sod, coord=(1.0, 2.0))
ion2 = Ion(ion_type=sod, coord=(2.0, 5.0))

print ion1.coord
print ion1.ion_type.name
print ion2.coord

not_an_ion_type = {'key': 'value'}

ion3 = Ion(ion_type=not_an_ion_type, coord=(2.0, 5.0))
</pre>

Which generates the expected output:

<pre>
(1.0, 2.0)
Sodium
(2.0, 5.0)
Traceback (most recent call last):
  File "test.py", line 23, in <module>
    ion3 = Ion(ion_type=not_an_ion_type, coord=(2.0, 5.0))
  File "/home/bjorn/vc/param/param/parameterized.py", line 866, in __init__
    self._setup_params(**params)
  File "/home/bjorn/vc/param/param/parameterized.py", line 782, in override_initialization
    fn(parameterized_instance,*args,**kw)
  File "/home/bjorn/vc/param/param/parameterized.py", line 1207, in _setup_params
    setattr(self,name,val)
  File "/home/bjorn/vc/param/param/__init__.py", line 746, in __set__
    self._check_value(val,obj)
  File "/home/bjorn/vc/param/param/__init__.py", line 743, in _check_value
    (self._attrib_name, self.class_.__name__, val))
ValueError: Parameter 'ion_type' value must be an instance of IonType, not '{'key': 'value'}'
</pre>