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.

components/sud.py

@@ -1,4 +1,3 @@
-import json
 from enum import Enum
 from utils.value import AttributeChange
 
@@ -46,16 +45,26 @@ def _build_step(number, defaults, raw_step):
 	return step
 
 
+EMPTY_SUD = {
+	'Name': '',
+	'Description': '',
+	'pot_mass': 0,
+	'pot_material': None,
+	'steps': [],
+}
+
+
 class Sud(AttributeChange):
-	def __init__(self, path):
+	def __init__(self):
+		"""Starts out with no schedule loaded - one of potentially several
+		sude/*.json files on disk is brought in later via load(), kept
+		purely in memory (see load()) until replaced or the server
+		restarts."""
 		AttributeChange.__init__(self)
-		self.path = path
-		with open(path) as f:
-			data = json.load(f)
-		self._data = data
+		self._data = EMPTY_SUD
 
 		(self.name, self.description, self.schedule,
-			self.pot_mass, self.pot_material) = self._parse_data(data)
+			self.pot_mass, self.pot_material) = self._parse_data(self._data)
 
 		self._paused_from = None
 		self._reset_run_state()
@@ -88,20 +97,17 @@ class Sud(AttributeChange):
 		self._paused_from = None
 
 	def save(self):
-		"""Returns the currently running sud.json document, for a client to
-		edit and hand back to load(). Purely in-memory - load() no longer
-		touches disk, so this is just whatever was last loaded (the file
-		given to __init__, or a later load()), not necessarily what's on
-		disk at self.path anymore."""
+		"""Returns the currently running sud.json document (EMPTY_SUD if
+		none has been loaded yet), for a client to edit and hand back to
+		load(). Purely in-memory - never read from or written to disk."""
 		return self._data
 
 	def load(self, data):
-		"""Replaces the running schedule with a new sud.json document (kept
-		in memory only - never written to disk, so the file originally
-		configured via self.path is never touched by this), resetting to
-		IDLE as if freshly loaded. A client wanting to persist a schedule
-		does so explicitly itself, e.g. by writing save()'s result wherever
-		it chooses. Refused while a brew is in progress, or if data is
+		"""Replaces the running schedule with a new sud.json document, kept
+		in memory only (never written to disk - a client wanting to persist
+		a schedule does so explicitly itself, e.g. by writing save()'s
+		result to one of the sude/*.json files), resetting to IDLE as if
+		freshly loaded. Refused while a brew is in progress, or if data is
 		malformed - in either case the current schedule is left untouched."""
 		if self.state not in (SudState.IDLE, SudState.DONE):
 			return False