Standardize time metadata

[samsrabin]

Description of changes

Standardizes a dimension name of output variable time_bounds (from hist_interval to nbnd), as well as attributes for that plus mcdate, mcsec, mdcur, and mscur.

Specific notes

Contributors other than yourself: @billsacks, @ekluzek

CTSM Issues Fixed: Resolves #1693.

Are answers expected to change (and if so in what way)? No.

Any User Interface Changes (namelist or namelist defaults changes)? No.

Testing performed

New metadata in a history file from a short test:

netcdf standardize-time-metadata_rtm.clm2.h0.1850-01-01-00000 {
dimensions:
...
    nbnd = 2 ;
...
variables:
...
    int mcdate(time) ;
        ...
        mcdate:calendar = "noleap" ;
    int mcsec(time) ;
        ...
        mcsec:calendar = "noleap" ;
    int mdcur(time) ;
        ...
        mdcur:calendar = "noleap" ;
    int mscur(time) ;
        ...
        mscur:calendar = "noleap" ;
...
    double time_bounds(time, nbnd) ;
        time_bounds:long_name = "time interval endpoints" ;
        time_bounds:units = "days since 1850-01-01 00:00:00" ;
        time_bounds:calendar = "noleap" ;

All aux_clm tests pass bit-for-bit against ctsm5.1.dev131, except for expected failures.

[samsrabin]

Marked as blocked:dependency because, at the moment, Externals.cfg points to branches of my personal MOSART (issue ESCOMP/MOSART#53) and RTM (issue ESCOMP/RTM#31) forks that contain the same changes. I haven't yet submitted those as pull requests, because I first wanted to see if any more testing is requested.

[samsrabin]

From CTSM software engineering meeting today: This doesn't need to wait on other time-related PRs. It can come in as soon as the relevant externals are updated (and Externals.cfg here is changed to point to them).