#5 Add overrides for model configurations

[Rick Hall]

The ChoiceMaker record-layout schema (*.schema) contains a section called productionProbabilityModels, which defines the model configurations that will be loaded the ProbabilityModel manager (PMManager) class. Each model configuration has a name plus a set of other attributes. It is often convenient to slightly tweak one configuration and refer to it by another name. Right now, there's way to refer to the same configuration by more than one name. As a result, configurations are often copied and then slightly tweaked, leading to a lot of duplication configuration information that's hard to maintain.

The idea would be to write a configuration like this:
<model name="some_parent_configration" file="some_path_to_a_file">
<parameter name="some_param" value="some_value"/>
<parameter name="some_other_param" value="some_other_value"/>
<...more model parameters...>
</model>
<model name="a_tweak" overrides="some_parent_configuration">
<parameter name="some_other_param" value="some_new_value"/>
</model>

In the example shown, the tweaked configuration (name="a_tweak") inherits all the attributes and parameters of the parent configuration (name="some_parent_configration") except for the "some_other_param" parameter, which it overrides with the "some_new_value" setting.