Add default_source class members pointing to ParamSource classes to all
ConfigurableParameter subclasses.
This is useful to automatically set up a default ParamSource for a given
ConfigurableParameter subclass, during user interaction to produce a
batch personalization.
For example, if the user selects a Pin1 parameter, a calling program can
implicitly set this to a RandomDigitSource, which will magically make it
work the way that most users need.
BTW, default_source and default_value can be combined to configure a
matching ParamSource instance:
my_source = MyParam.default_source.from_str( MyParam.default_value )
Change-Id: Ie58d13bce3fa1aa2547cf3cee918c2f5b30a8b32
1c622a6101