Generalize FastIsostasy restarts #197

PR #183 added support for FastIsostasy, but restarts are not fully confirmed to BFB and only a single restart file that gets sequentially updated is supported for FI. For production runs, we want the ability to restart from an arbitrary MALI restart file, not just the most recent one. That functionality could be added to FI by having it write restart files with xtime in the filename.

Requirements:

• add a datetime stamp following xtime format to FI restart file names for when the restart filename is passed to FI (https://github.com/MALI-Dev/E3SM/pull/183/changes#diff-2f6ec4b2448bb009059874f9424f7d902e40ead469b700a5a0ba3be60c22b510R1330-R1331)
• make use of existing restart alarms if possible to avoid duplication of timekeeping in different parts of the code
• avoid changes to mpas framework or use of low level framework and streams functionality if possible
• automatic handling of filename for both input and output of restart stream
• include useful error messages when files are missing are other i/o issues are encountered
• handle the fact that FI runs on a single processor but the rest of MALI uses MPI