Create a range of nlPeriods. Autodetects the type of nlPeriod and creates a
character vector of nlPeriods filling in the intermediate nlPeriods.
NOTE: Both start and end range must be valid and of the same type i.e.
"OLS" years or "VIIRS" yearMonths
# NOT RUN {#get OLS years between 2004 and 2010nlRange("2004", "2010")
#get VIIRS yearMonths between Apr 2012 and Dec 2014nlRange("201204", "201412")
# }