Add timestamp to rpointer files, but make it backwards compatible with files without the timestamp

[ekluzek]

CESM is going to a system wide change where the restart rpointer text files will have the simulation timestamp in the filename. However, the model will still be backwards compatible to use the older filenames without it. But, new rpointer files created by the model will have the timestamp in the filename.

PR's for CTSM and MOSART for this work are here:

ESCOMP/CTSM#2757
ESCOMP/MOSART#105

[slevis-lmwg]

@ekluzek when ready to merge this to master, remember that I merged to master the ChangeLog update without making a new tag.

#56