Modify naming and attributes of time variables on history files to be consistent with other CESM components

[phillips-ad]

These changes are being requested to improve the ease-of-access of CESM data and to normalize the look of the metadata across CESM components. See ESCOMP/CESM#194 and especially the google doc referenced from ESCOMP/CESM#194 (comment).

Current POP time array output:

double time(time) ;
	time:long_name = "time" ;
	time:units = "days since 0000-01-01 00:00:00" ;
	time:bounds = "time_bound" ;
	time:calendar = "noleap" ;
double time_bound(time, d2) ;
	time_bound:long_name = "boundaries for time-averaging interval" ;
	time_bound:units = "days since 0000-01-01 00:00:00" ;

Current MOM time array output:

double time(time) ;
	time:long_name = "time" ;
	time:units = "days since 0001-01-01 00:00:00" ;
	time:cartesian_axis = "T" ;
	time:calendar_type = "NOLEAP" ;
	time:calendar = "NOLEAP" ;
	time:bounds = "time_bnds" ;
double time_bnds(time, nv) ;
	time_bnds:long_name = "time axis boundaries" ;
	time_bnds:units = "days" ;
	time_bnds:missing_value = 1.e+20 ;
	time_bnds:_FillValue = 1.e+20 ;

Proposed MOM time array output:

double time(time) ;
	time:long_name = "time" ;
	time:units = "days since 0001-01-01 00:00:00" ;
	time:bounds = "time_bounds" ;
	time:calendar = "noleap" ;
double time_bounds(time, nbnd) ;
	time_bounds:long_name = "time interval endpoints" ;
	time_bounds:units = "days since 0001-01-01 00:00:00" ;
	time_bounds:calendar = "noleap" ;

Changes made: time_bnds-> time_bounds, change 2nd dimension name of time_bounds to nbnd, change calendar attribute to noleap, remove time@calendar_type and time@cartesian_axis, change time_bounds@long_name = “time interval endpoints”, change time_bounds@units to match time@units, set time_bounds@calendar = “noleap”, and remove time_bounds@missing_value and time_bounds@_FillValue.

@billsacks @phillips-ad @strandwg are filing similar issues for each component.