Improving the xvik program

[emillour]

Some suggestions for improving "xvik":

• add comment (and/or make them be in English rather than French)
• add in the output files not only the instantaneous values by also diurnal mean (which is what one most often checks and compare to observations)
• when running one year or more the last time step of the year (i.e. at sol=669.0 modulo 669) is put as first element (time=0.0) in the file for the next year, often leading to an orphan file of 1 element... maybe put it as the final element in the previous year file.
• add output files for Insight?