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.

scripts/demos/sud/demo_sud.py

@@ -1,3 +1,4 @@
+import json
 import numpy as np
 from matplotlib.pyplot import plot, step, figure, subplot, grid, show, legend, yticks
 from components.sud import Sud, SudState
@@ -33,7 +34,9 @@ if __name__ == '__main__':
 		}
 	}
 
-	sud = Sud("sude/sud_0010.json")
+	sud = Sud()
+	with open("sude/sud_0010.json") as f:
+		sud.load(json.load(f))
 
 	first_step = sud.schedule[0]
 	plant_params = {