MLECO-3164: Additional refactoring of KWS API

Part 1
* Add KwsClassifier
* KwsPostProcess can now be told to average results
* Averaging is handlded by KwsClassifier
* Current sliding window index is now an argument of DoPreProcess

Change-Id: I07626da595ad1cbd982e8366f0d1bb56d1040459
diff --git a/source/application/api/use_case/ad/include/AdMelSpectrogram.hpp b/source/application/api/use_case/ad/include/AdMelSpectrogram.hpp
index 05c5bfc..b8a9dfc 100644
--- a/source/application/api/use_case/ad/include/AdMelSpectrogram.hpp
+++ b/source/application/api/use_case/ad/include/AdMelSpectrogram.hpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021 Arm Limited. All rights reserved.
+ * Copyright (c) 2021-2022 Arm Limited. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
@@ -41,7 +41,7 @@
         {}
 
         AdMelSpectrogram()  = delete;
-        ~AdMelSpectrogram() = default;
+        virtual ~AdMelSpectrogram() = default;
 
     protected: