createCityProcessor: Create closure to identify and aggregate heat waves
Description
This function creates a closure that returns a
   dataframe with the identified heat waves and heat wave characteristics for
   a given city for the specified projection period, as generated by the
   createHwDataframe function.
Usage
createCityProcessor(global)
Arguments
global
An list object created by gen_hw_set that
includes user specifications (e.g., the path to the output directory, the
path to the input climate projections, the dataframe with city
locations). Value
This function creates a closure that takes inputs of threshold,
   ensembleSeries, and custom and will
   find and characterize all heat waves in all cities for a given
   ensemble. See the help file for formHwFrame for more
   information on the format of the dataframe created by this closure.