monitor: tweak/simplify Progress

Tweak the Progress class to be simpler. Given that progress does
not need to support arbitrary depth but only has a single level
the class now just exposes "sub_progress" to the caller.

When the main progress is advanced the sub_progress is now fully
deleted instead of just reset. The rational is that when the main
progress is done and advances a step it is very likely that a
new sub_progress is required and it's most likely an error if
the same sub_progress will get re-used.

This means that `reset()` can be removed as it's not used anymore
(and YAGNI). We can add it back when we have a use-case.

It also change the code so that "total" starts with 0 instead
of `None` (principle of least surprise). This means that now
`progress.incr()` is called in the JSONSeqMonitor() for
`finish()` and `result()` to indicate that the pipeline/stage
is finished.

test/mod/test_monitor.py

@@ -160,25 +160,27 @@ def test_context():
 
 def test_progress():
     prog = Progress("test", total=12)
-    subprog = Progress("test-sub1", total=3)
-    prog.sub_progress(subprog)
-    assert prog.done is None  # starts with None until the first incr()
-    prog.incr()
-
-    prog.incr(depth=1)
+    prog.sub_progress = Progress("test-sub1", total=3)
+    # we start empty
     progdict = prog.as_dict()
     assert progdict["done"] == 0
     assert progdict["progress"]["done"] == 0
 
-    prog.incr(depth=1)
+    # incr a sub_progress only affect sub_progress
+    prog.sub_progress.incr()
     progdict = prog.as_dict()
     assert progdict["done"] == 0
     assert progdict["progress"]["done"] == 1
 
+    prog.sub_progress.incr()
+    progdict = prog.as_dict()
+    assert progdict["done"] == 0
+    assert progdict["progress"]["done"] == 2
+
     prog.incr()
     progdict = prog.as_dict()
     assert progdict["done"] == 1
-    assert progdict["progress"]["done"] is None, "sub-progress did not reset"
+    assert progdict.get("progress") == None, "sub-progress did not reset"
 
 
 def test_json_progress_monitor():