Sud: server always starts empty; sude/*.json is purely a client concern

Several sude/*.json schedules can exist on disk; only one runs at a
time, chosen by a client Load. Sud no longer takes a path at all - it
starts empty (EMPTY_SUD) and the server config no longer names a
specific schedule file, removing the implicit link to sud_0010.json
in particular. Reading/writing sude/*.json is now entirely the
client's job (e.g. the GUI's Load/Save file dialogs); the server's Sud
only ever holds whatever was last Load()ed, in memory, until replaced
or the server restarts.

Updates demo_sud.py/demo_sud_save_load.py to construct an empty Sud()
and load() a schedule explicitly, matching the new constructor.

config.json.sim

@@ -1,6 +1,5 @@
 {
 	"ambient_temperature": 20,
-	"sud": "sude/sud_0010.json",
 	"Controller" : {
 	"dt": 0.1,
 	"sim_warp_factor": 10.0,