Added default parameter option for materials class

The material model class can now accept default parameters
instead of just zeroing out the entire UI array. It is done
in this way:

...
class MyModel(Material):
    name = "mymodel"
    param_names = ["K", "G", "Y"]
    param_defaults = [1.0e10, 0.3e10, "1.0e99"]
    ...

The list 'param_defaults' gets turned into a numpy array
and the user inputs are put into that. If the MyModel class
does not have an attribute named "param_defaults", the previous
behavior is used (where the UI array is zeroed out).

All tests pass (Debian 64-bit)