title: how to read this netCDF file in python?
body: <p>I have this NetCDF file.</p>
  
  <p>it has 2 groups in it! I want to select one group and get the desired variable. I tried this in multiple ways but I failed. I could use simply   </p>
  
  <pre><code>print(ncfile.variables)
  </code></pre>
  
  <p>but it's not working!</p>
  
  <p>the file structure looks like this, i want to chose group /O4/ and variable 'aerosol_optical_depth'</p>
  
  <pre><code>filename =
      'output.nc'
  Source:
             C:\Users\IITM\Desktop\work\output.nc
  Format:
             netcdf4
  Global Attributes:
             _NCProperties = 'version=2,netcdf=4.6.3,hdf5=1.10.2'
             version       = 1
  Dimensions:
             DIM_SCAN_NAME  = 3
             DIM_ANGLE_NAME = 10
             DIM_LAYERS     = 20
  Groups:
      /O4/
          Variables:
              vertical_column_density                                 
                     Size:       3x1
                     Dimensions: /DIM_SCAN_NAME
                     Datatype:   double
                     Attributes:
                                 _FillValue  = NaN
                                 description = 'o4 vertical column density'
                                 units       = 'molec/cm5'
              aerosol_optical_depth                                   
                     Size:       3x1
                     Dimensions: /DIM_SCAN_NAME
                     Datatype:   single
                     Attributes:
                                 _FillValue  = NaN
                                 description = 'Total aerosol optical depth'
                                 units       = '1'
  
   /TG/
          Variables:
              vertical_column_density                                  
                     Size:       3x1
                     Dimensions: /DIM_SCAN_NAME
                     Datatype:   single
                     Attributes:
                                 _FillValue  = NaN
                                 description = 'gas vertical column density'
                                 units       = 'molec/cm2'
              vertical_column_density_error                            
                     Size:       3x1
                     Dimensions: /DIM_SCAN_NAME
                     Datatype:   single
                     Attributes:
                                 _FillValue  = NaN
                                 description = 'gas vertical column density error calculated from covariance smoothing error matrix, covariance measurement noise error matrix and systematic error as a fixed fraction of vcd'
                                 units       = 'molec/cm2'
  </code></pre>