Support append to `ElectrodeTable`

[oruebel]

Aappending new rows to the ElectrodesTable after it has been finalized does currently not work correctly. I.e., currently the ElectrodesTable still has to be created in one shot. #209 addressed finalization of ElectrodeTable to support multiple calls to ElectrodeTable.finalize. Appending to location and group_name should in principal already work, as DynamicTable.addColumn was updated in #209 to append values to a column if it already exists. However, the following issues remain:

• DynamicTable.addReferenceColumn does currently not support appending of values, i.e., appending values after finalize will result in invalid behavior
• When an ElectrodeTable (or any DynamicTable) is being read from file, then the VectorData should be read from file to make sure we append to existing data. We should check that this actually happens.
• finalize should be save to call multiple times on both DynamicTable and ElectrodeTable. Add test to confirm that ElectrodeTable::finalize() can be called again when appending to an existing table.

[oruebel]

We could add a function DynamicTable::getColumn to retrieve or construct a VectorData for a column from the DynamicTable and in turn remove the caching of individual VectorData in the ElectordesTable (i.e., std::shared_ptr<VectorData> m_groupNamesVectorData; and std::shared_ptr<VectorData> m_locationsVectorData;) to move the management of columns fully into the DynamicTable base class. The function to access these would probably look something like this:

/**
   * @brief Get the `VectorData` object for a given column name if it exists
   */
  inline std::shared_ptr<VectorData> getColumn(const std::string& colName)
  {
    // Iterate through the m_recordingColumns to find the column with the
    // specified name
    auto columnObjs = m_recordingColumns->getRecordingObjects();
    for (SizeType i = 0; i < columnObjs.size(); ++i) {
      // compare name of the columns
      if(columnObjs[i]->getName() == colName) {
        return std::dynamic_pointer_cast<VectorData>(columnObjs[i]);
      }    
    }

    // If not found in m_recordingColumns, check if it exists in the file
    std::string columnPath = AQNWB::mergePaths(m_path, colName);
    auto ioPtr = getIO();
    if (ioPtr != nullptr) {
      if (ioPtr->objectExists(columnPath)) {
        if (ioPtr->getStorageObjectType(columnPath)
            == StorageObjectType::Dataset)
        {
          if(colName == "id")
          {
            return std::dynamic_pointer_cast<VectorData>(m_rowElementIdentifiers);
          }
          else
          {
            auto columnVectorData = VectorData::create(columnPath, ioPtr);
            if (columnVectorData) {
              m_recordingColumns->addRecordingObject(columnVectorData);
            }
            return columnVectorData;
          }
        }
      }
    } else {
      std::cerr << "IO object has been deleted. Can't get column: " << colName
                << " in DynamicTable: " << m_path << std::endl;
    }
    return nullptr;
  }

[oruebel]

Related to #94