CLIMATE-483 - Add basic PatternCorrelation metric

- Rename old PatternCorrelation metric to TemporalPatternCorrelation
  since it does an annual reshaping of the datasets before doing a
  comparison and that isn't adequately reflected in the metric's name.
  The new name also falls in line with the functionality of the
  TemporalStdDevRatio metric which also does such a rebin.
- Add basic PatternCorrelation metric that uses
  scipy.stats.mstats.linregress to return the r_value for the reference
  and target datasets. Note, the metric no longer returns a tuple of
  values. It now only returns the r_value. This falls much more in line
  with the metrics name in my opinion.