title: Calculate centroid of entire GeoDataFrame of points
body: <p>I would like to import some waypoints/markers from a geojson file. Then determine the centroid of all of the points. My code calculates the centroid of each point not the centroid of all points in the series. How do I calculate the centroid of all points in the series?</p>
  <pre><code>import geopandas
  
  filepath = r'Shiloh.json'
  
  gdf = geopandas.read_file(filepath)
  
  xyz = gdf['geometry'].to_crs('epsg:3587')
  
  print(type(xyz))
  print(xyz)
  
  # xyz is a geometry containing POINT Z
  
  c = xyz.centroid
  
  
  # instead of calculating the centroid of the collection of points
  # centroid has calculated the centroid of each point. 
  # i.e. basically the same X and Y data as the POINT Z.
  
  </code></pre>
  <p>The output from print(type(xyz)) and print(xyz)</p>
  <pre><code>&lt;class 'geopandas.geoseries.GeoSeries'&gt;
  0    POINT Z (2756810.617 248051.052 0.000)
  1    POINT Z (2757659.756 247778.482 0.000)
  2    POINT Z (2756907.786 248422.534 0.000)
  3    POINT Z (2756265.710 248808.235 0.000)
  4    POINT Z (2757719.694 248230.174 0.000)
  5    POINT Z (2756260.291 249014.991 0.000)
  6    POINT Z (2756274.410 249064.239 0.000)
  7    POINT Z (2757586.742 248437.232 0.000)
  8    POINT Z (2756404.511 249247.296 0.000)
  Name: geometry, dtype: geometry
  </code></pre>
  <p>the variable 'c' reports as (centroid of each point, not the centroid of the 9 POINT Z elements) :</p>
  <pre><code>0    POINT (2756810.617 248051.052)
  1    POINT (2757659.756 247778.482)
  2    POINT (2756907.786 248422.534)
  3    POINT (2756265.710 248808.235)
  4    POINT (2757719.694 248230.174)
  5    POINT (2756260.291 249014.991)
  6    POINT (2756274.410 249064.239)
  7    POINT (2757586.742 248437.232)
  8    POINT (2756404.511 249247.296)
  dtype: geometry
  </code></pre>