Allow a Sud step to combine a ramp and a hold

A step can now carry both 'ramp' and 'hold': it ramps to the target
temp, then holds there for the given duration, instead of needing two
separate schedule entries. Sud.temp_reached() switches such a step
from its ramp phase into its hold phase in place (re-firing the
step-changed callback so SudTask/demo_sud re-apply the hold's stirrer
settings); user_wait_for_continue still applies once the whole step -
both phases - is done.

Merges sud_0010.json's three ramp/hold pairs into combined steps.

README.md

@@ -129,13 +129,19 @@ environment (`$WORKON_HOME`/`$BREWPI_HOME`) on a Raspberry Pi-style deployment.
 ## Mash schedules
 
 Files under `sude/` describe a brew's mash schedule ("Sud"): `pot_mass`,
-`pot_material` (fixed for the whole brew), and a `steps` list, each either:
+`pot_material` (fixed for the whole brew), and a `steps` list, each a ramp, a
+hold, or both:
 
 - a ramp — `"ramp": {"rate": ..., "temp": ...}` — ramp to `temp` at `rate`
-  (°C/min), or
+  (°C/min), and/or
 - a hold — `"hold": {"duration": ...}` — hold the current target for
   `duration` minutes (omit/`0` for an immediate step).
 
+A step with both ramps to `temp` and then holds there for `duration` -
+useful for a mash rest ("ramp to 63°C, then hold 40 min") without needing two
+separate schedule entries. `user_wait_for_continue`/`user_message` (below)
+apply once the whole step is done, i.e. after the hold phase if there is one.
+
 Both `ramp` and `hold` carry a `stirrer` block (`speed`, `interval_time`,
 `on_ratio`): `interval_time: 0` runs the stirrer continuously at `speed`;
 `interval_time > 0` pulses it on a period of `interval_time` seconds, on for