Refperiodparsing (fixes #74)

Changes the parsing of the time period to [start_time, end_time+frequency[. That means that one unit of the frequency in the time format will be added to the end_time string. For e.g. end_time '2010' with format 'YYYY' will become end_time + 1year, i.e. 2011,1,1,0,0,0. The upper time limit is not included in the time range (denoted by [ ), in other words start_time <= time < end_time+frequency. This change will make sure that indicating a period like 1981-2010 will extract data from 1981-01-01 to 2010-12-31 instead of until 2009-12-31 as it was before.