Create a range of nlPeriods. Returns a list of character vectors of
nlPeriods filling in the intermediate nlPeriods.
NOTE: Both start and end range must be valid and of the same type.
# NOT RUN {#get OLS years between 2004 and 2010nlRange("2004", "2010", "OLS.Y")
#get VIIRS yearMonths between Jan 2014 and Dec 2014nlRange("201401", "201412", "VIIRS.M")
# }