Update model output schema to dictate the requirements for time & time_bnds

[joshuatorrance]

There's been discussions elsewhere on unifying the handling of time & time_bnds. The current proposal is for the files to follow this format:

double time(time) ;
    time:axis = "T" ;
    time:bounds = "time_bnds" ;
    time:calendar = "proleptic_gregorian" ;
    time:long_name = "time" ;
    time:standard_name = "time" ;
    time:units = "days since 0001-01-01 00:00" ;
double time_bnds(time, bnds) ;

This can be enforced by adding time and time_bnds to the variable portion of the output file schema.