add String method (#552)

* all: add String() method for improved log output

dnn_string.go

@@ -0,0 +1,35 @@
+package gocv
+
+func (c NetBackendType) String() string {
+	switch c {
+	case NetBackendDefault:
+		return ""
+	case NetBackendHalide:
+		return "halide"
+	case NetBackendOpenVINO:
+		return "openvino"
+	case NetBackendOpenCV:
+		return "opencv"
+	case NetBackendVKCOM:
+		return "vulkan"
+	}
+	return ""
+}
+
+func (c NetTargetType) String() string {
+	switch c {
+	case NetTargetCPU:
+		return "cpu"
+	case NetTargetFP32:
+		return "fp32"
+	case NetTargetFP16:
+		return "fp16"
+	case NetTargetVPU:
+		return "vpu"
+	case NetTargetVulkan:
+		return "vulkan"
+	case NetTargetFPGA:
+		return "fpga"
+	}
+	return ""
+}