title: Calculating correlation between 2 cubes on different grids
body: <p>I have 2 cubes which have different grid and I tried to directly calculate correlation between them using:
  <code>iris.analysis.stats.pearsonr(cube1, cube2)</code>
  This returned an error:</p>
  <blockquote>
  <p>ValueError: Insufficient matching coordinate metadata to resolve cubes, cannot map dimensions (1, 2) of the RHS cube 'unknown' to the LHS cube 'unknown'.</p>
  </blockquote>
  <p>I thought that the error above could be caused by different grid and shape of the cubes. Cubes 1 and 2 are GCM and ERA5 datasets. I think calculating correlation without regridding the datasets wouldn't be right, because ERA5 has a regular grid while the GCM doesn't so there's no one-to-one match of the datapoints. I regridded the cubes to the GCM grid but when I tried to calculate correlation again I got the same error.</p>
  <p>What am I doing wrong? Could anyone help me solve this?</p>