Reenforcement of CF compliance for time operation

[ymipsl]

1) time units must be conform to UDUnits specification ( ​http://www.unidata.ucar.edu/software/udunits/udunits-2.1.24/udunits2.html#Database )

2) Add cell methods for time operation in field metadata
cell_methods = “time_variable_name: online_operation (interval: interval_operation)
ex :

float sosstsst(time_counter, y, x) ;

sosstsst:long_name = "sea surface temperature" ;
sosstsst:units = "degC" ;
sosstsst:online_operation = "average" ;
sosstsst:interval_operation = "5760s" ;
sosstsst:interval_write = "1mo" ;
sosstsst:_FillValue = 1.e+20f ;
sosstsst:missing_value = 1.e+20f ;
sosstsst:coordinates = "time_centered nav_lon nav_lat" ;
sosstsst:cell_methods = "time_centered: mean (interval: 5760s)"

We have a name mapping between xios and Cf compliance :
average -> mean
instant -> point
accumulate -> sum

[rlacroix]

Implemented in r612.