title: How to wrap nested string (metadata) into TagGroup
body: <p>My imported metadata has a pre-defined nested structure (an example show in the following), which is a single string after imported to DM. 
  The whole metadata and each branch level are included in brace marks {}, all the key and key values are included by quotation marks "" and deliminated by colon :</p>
  
  <p>My question is, how to convert the data and wrap them into a TagGroup object so that indexing, searching and data accessing operations can be done much easier?</p>
  
  <p>Thanks!</p>
  
  <p>Here is an example:</p>
  
  <pre><code>{
      "Acquisition": {
          "AcquisitionStartDatetime": {
              "DateTime": "1473763749"
          },
          "AcquisitionDatetime": {
              "DateTime": "0"
          },
          "BeamType": "",
          "SourceType": "Monochromator"
      },
      "BinaryResult": {
          "AcquisitionUnit": "",
          "CompositionType": "",
          "DetectorIndex": "3",
          "Detector": "HAADF",
          "PixelSize": {
              "width": "5.408370946750477e-010",
              "height": "5.408370946750477e-010"
          },
          "PixelUnitX": "m",
          "PixelUnitY": "m",
          "Offset": {
              "x": "-2.769085924736244e-007",
              "y": "-2.769085924736244e-007"
          },
          "Encoding": ""
      },
      "Sample": "",
      "GasInjectionSystems": ""
  }
  </code></pre>