Merge pull request #3427 from gregchapman-dev/gregc/single_S_turns

iohumdrum.cpp rejects valid single-character turns (e.g. "4aS").   That is good.   I probably suppressed single-letter turns to promote encoding with interval information.    I will probably add a warning when this happens (and maybe make the turn red in the notation to do that.

src/iohumdrum.cpp

@@ -25693,7 +25693,7 @@ void HumdrumInput::addTurn(hum::HTp token, const string &tok, int noteIndex)
         }
     }
 
-    if (turnstart == turnend) {
+    if (turnstart == -1) {
         return;
     }
     std::string turnstr = tok.substr(turnstart, turnend - turnstart + 1);