ERT-699: gen_data_config refactoring + testing I

Have refactored the functionality to create and initialize the
gen_data_config nodes.
ERT-699: gen_data_config refactoring + testing II
ERT-699: gen_data_config refactoring + testing III
ERT-699: gen_data_config refactoring + testing IV

Have added the REPORT_STEPS:xxx attribute as required for the GEN_DATA
configuration.
ERT-699: Updated enkf_config_node->internalize vector