ERT-651 Minor fs read check fixup
ERT-651 Added python tests for GenDataConfig, to test the active_mask when swapping cases. Updated active_list representation to be valid also when no _active file exists on disk.
ERT-651 Added enkf_fs_type as argument to enkf_node implementations, need this in gen_data to read/write the correct active_mask representation.
ERT-651 ASCII art upgrade
ERT-651 Adds enkf_fs instance to observation nodes, to be able to find active mask for gen_obs dynamically.