partial subscriptions

not really partial. they just happen per peripheral now

telemetry/virtual_field.go

@@ -0,0 +1,9 @@
+package telemetry
+
+// VirtualField will derive data from telemetry primitives
+// So fuel per lap predictions, compound gauge lights and so on
+type VirtualField interface {
+	Name() string
+	Process(td *TelemetryData)
+	EnsureSubscribed() []FieldID
+}