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 RTM time array output:

double time(time) ;
	time:long_name = "time" ;
	time:units = "days since YYYY-MM-DD HH:MM:SS" ;
	time:calendar = "noleap" ;
	time:bounds = "time_bounds" ;
double time_bounds(time, hist_interval) ;
	time_bounds:long_name = "history time interval endpoints" ;

Proposed RTM time array output:

double time(time) ;
	time:long_name = "time" ;
	time:units = "days since YYYY-MM-DD HH:MM:SS" ;
	time:calendar = "noleap" ;
	time:bounds = "time_bounds" ;
double time_bounds(time, nbnd) ;
	time_bounds:long_name = "time interval endpoints" ;
	time_bounds:units = "days since 2015-01-01 00:00:00" ;
	time_bounds:calendar = "noleap" ;

Changes made: add units/calendar attributes to time_bounds, modify time_bounds@long_name, change 2nd dimension name of time_bounds to nbnd

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