restructure class "SilecsCluster" and rename it to "SilecsDesign"

• via SilecsService a client sometimes wants to connect to a controller(plc), not to a SilecsDesign
• The SilecsPLC should ref one to many "SilecsDesign" which than ref one to many "SilecsDevice" ... but anyhow "SilecsPLC" needs to be able to search devices by name (devicename unique per plc).
• "SilecsDesign" still needs to have "void recv(std::string blockName);" and void "send(std::string blockName);" to be able to speed up comm in Block-Mode

If a FESA-class is spread over multiple PLC's, it should be no problem to ask "SilecsService" either for a "SilecsPLC", or for a "SilecsDesign"