title: Xarray append values in dataset along coordinate
body: <p>Trying to append values to a dataset 
  I need to create a blank xarray dataset with two+ dimensions where one is fixed and the other is incrementing to two+ output sets</p>
  
  <p>The code example is something like</p>
  
  <pre><code>import numpy as np
  import xarray as xr
  
  #fixed coordinte diminsion 
  time=np.linspace(0, 4*np.pi)
  
  #dummy data to insert
  #ith is the moving coordainte 
  amp={i:i*np.sin(time) for i in range(0, 10)}
  phase={i:i*np.cos(time) for i in range(0, 10)}
  
  #create the xr dataset
  Data=xr.Dataset(coords={'time':time, 'ith':0})
  #create the holding loctions to incoming data
  Data['amp']=None; Data['phase']=None
  
  
  #now need to increase i in the dataset and append data
  #and this is where I am stuck
  for i in range(0, 10):
     Data['amp'].loc[dict(ith=i)]=amp[i]
     Data['phase'].loc[dict(ith=i)]=phase[i]
  </code></pre>
  
  <p>This would be useful for taking data from a weather sensor over 24hours each day of the year and appending to the dataset</p>