Customise the interface by overloading the default commands

NA62DimCommand: freed from references to ConfigDecoder, default behavior
is waiting for file after every command
	The "Command" command methods can be overloaded in the derived class if
the default behavior is not desire (or must be extended, still possible
to call the default behavior)
	The decodeFile method of the "FileContent" command can be overloaded

NA62DimServer: If specified with initCommands, overload the default
command. Pass NULL pointer for each default command that must be used.

TestServer uses the new NA62DimServer interface
Hide fNextState Getter and Setter. 

Use instead waitConfigurationFile(expectedState) and
moveToExpectedState(). Use setState to explicitely go to specific state
and cancel nextState.