xarray output for power spec and rank hist metrics

[14renus]

output of power spec and rank histogram is xarray:

• remove option for return_raw_dict, should return xarray if needed all metrics

xarray needs to have var has data variable, since some variables have level dimension but not all:

• cannot make metric as the data variable in xarray

fix plot:

• read xarray format with metric as extra dimension
• select extra optional dimensions such as level for certain variables
• alias for variable/level in plot title

match rankhist plot to paper:

• rankhist log frequency
• option for color
• x axis label