Add a convenience func to retrieve the internal attr. hist.

A new function that can be imported to read the internal attribute history. The history is conveniently converted to a numpy.array.

What is missing: I could also extract the time stamps but did not know how to add that to the retrieved array. Could be done later.

CDB/test_ConfigDb.json

+{
+    "servers": {
+        "PCC": {
+            "1": {
+                "PCC": {
+                    "LTS/PCC/1": {
+                        "properties": {
+                            "OPC_Server_Name": [
+                                "ltspi.astron.nl"
+                            ]
+                        }
+                    }
+                }
+            }
+        },
+        "SDP": {
+            "1": {
+                "SDP": {
+                    "LTS/SDP/1": {
+                        "properties": {
+                            "OPC_Server_Name": [
+                                "DESPi2.astron.nl"
+                            ]
+                        }
+                    }
+                }
+            }
+        },
+        "example_device": {
+            "1": {
+                "example_device": {
+                    "LTS/example_device/1": {
+                         "attribute_properties": {
+                            "Ant_mask_RW": {
+                                "archive_period": [
+                                    "600000"
+                                ]
+                            }
+						},
+                        "properties": {
+                            "OPC_Server_Name": [
+                                "host.docker.internal"
+                            ],
+                            "OPC_Server_Port": [
+                                "4842"
+                            ],
+                            "OPC_Time_Out": [
+                                "5.0"
+                            ]
+                        }
+                    }
+                }
+            }
+        }
+    }
+}