Allow get_param_values() to work for Parameterized classes?

[ceball]

I think it would be reasonable for Parameterized.get_param_values() to work for a class as well as an instance. What do you think?

>>> class A(param.Parameterized):
...    x = param.Number(1)

>>> a = A()
>>> a.get_param_values()
[('name', 'A00009'), ('x', 1)]

>>> A.get_param_values()
[...]
TypeError: get_param_values() missing 1 required positional argument: 'self'

Should be a straightforward change to use @bothmethod. I guess onlychanged=True would only make sense for an instance, but I don't think it matters much.

[jbednar]

Sounds good!