Warnings and documentation about expected stream nesting

[jacobtylerwalls]

Starting a thread to stash some ideas for documenting better the expected (tested) nested stream hierachy of (Opus) - Score - Part - Measure - (Voice)

Here's a warning we could emit on show or write:

index c43979d30..12c9855d1 100644
--- a/music21/base.py
+++ b/music21/base.py
@@ -2575,6 +2575,9 @@ class Music21Object(prebase.ProtoM21Object):
 
         scClass = common.findSubConverterForFormat(regularizedConverterFormat)
         formatWriter = scClass()
+        if self.isStream and not self.isWellFormedNotation():
+            environLocal.warn('''Your operations on this stream have left it malformed. 
+Try running makeNotation(), or see docs for stream.isWellFormedNotation()''')
         return formatWriter.write(self,
                                   regularizedConverterFormat,
                                   fp=fp,
@@ -2642,6 +2645,9 @@ class Music21Object(prebase.ProtoM21Object):
 
         scClass = common.findSubConverterForFormat(regularizedConverterFormat)
         formatWriter = scClass()
+        if self.isStream and not self.isWellFormedNotation():
+            environLocal.warn('''Your operations on this stream have left it malformed. 
+Try running makeNotation(), or see docs for stream.isWellFormedNotation()''')
         return formatWriter.show(self,
                                  regularizedConverterFormat,
                                  app=app,

Somewhere in the user's guide wouldn't be a bad idea, either. Maybe a couple more examples in the doctests for stream.isWellFormedNotation.

This comes up constantly on the list, as you know.