IVGCVSW-2296 Add ParsePad method to TfParser

* Also added unit test armnnTfParser/test/Pad.cpp
* Added missing 'Pad' entry to GetLayerTypeAsCString(LayerType)
* Fixed the RefLayerSupport.IsPadSupported, now it returns true
* Small fix in Optimize. Only resolve stringstream to string once

Change-Id: Ieaa1886858a48cd761ac5f30454f73e44bdd4b8f
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8b351ad..9845124 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -397,6 +397,7 @@
             src/armnnTfParser/test/MaximumForLeakyRelu.cpp
             src/armnnTfParser/test/Minimum.cpp
             src/armnnTfParser/test/Multiplication.cpp
+            src/armnnTfParser/test/Pad.cpp
             src/armnnTfParser/test/MultiOutput.cpp
             src/armnnTfParser/test/PassThru.cpp
             src/armnnTfParser/test/Pooling.cpp