Output functions for the SWAT hydrological model

This function generates a file with observed values of streamflows for being used as input to SWAT-CUP (http://www.eawag.ch/organisation/abteilungen/siam/software/swat/index_EN)

1	obs2swatcup(x, tstep="daily", from, to, date.fmt="%Y-%m-%d", fname="Observations", dates, version=2, var.id="q_1")

`x`	zoo object with flow measurement(s) in 'm3/s'. It could be 'daily' or 'monthly'
`tstep`	time step in which are stored the values of 'x'. Valid values are: 'daily, 'monthly'
`from`	Character indicating the starting date for the values stored in all the files that will be read. It HAs to be in the format indicated by 'date.fmt'
`to`	Character indicating the starting date for the values stored in all the files that will be read. It HAs to be in the format indicated by 'date.fmt'
`date.fmt`	Character indicating the date format in which you provide 'from' and 'to', e.g. "
`fname`	Filename (without extension) of the output file. The number of observations will be added at the end of the filename
`dates`	OPTIONAL (not implmented yet). Vector with the dates to be used for converting a raw 'x' values into a 'zoo' object
`version`	integer indicating the version of SWAT-CUP for which the file has to be written
`var.id`	OPTIONAL. Only needed when 'version==3'. Character that have to be added as prefix before the year and day/month as second column in the observation file