Move networkdelay to plugins folder (#759)

* Move to plugins folder

* Rename dapp.go to plugin.go

plugins/research.go

 package plugins
 
 import (
-	"github.com/iotaledger/goshimmer/dapps/networkdelay"
 	analysisclient "github.com/iotaledger/goshimmer/plugins/analysis/client"
 	analysisdashboard "github.com/iotaledger/goshimmer/plugins/analysis/dashboard"
 	analysisserver "github.com/iotaledger/goshimmer/plugins/analysis/server"
+	"github.com/iotaledger/goshimmer/plugins/networkdelay"
 	"github.com/iotaledger/goshimmer/plugins/prometheus"
 	"github.com/iotaledger/goshimmer/plugins/remotelog"
 	"github.com/iotaledger/hive.go/node"