Consider removing get_background_pddl() API function and replace with get_pddl_domain_file()

[dtdannen]

I believe all of the information that would have been provided by the get_background_pddl() API function call can instead just be included under the pddl "(:constants" section of the domain file. In the future we will likely have multiple domain files for different versions of pddl, so we could change this API function call to get_pddl_domain_file().