Description
This PR allows the user to change the time used to set the filename when history files are written periodically using the diag_table.

The allowed options are "begin", "middle", and "end". If no option is present the default, "middle", will be used.

Fixes #626

How Has This Been Tested?
Passes diag_manager unit test.

"ocn_middle%4yr%2mo%2dy%2hr", 6, "hours", 1, "hours", "time", 6, "hours", "2 1 1 0 0 0", 0, "", "middle"
generates the files:
ocn_middle_0002_01_01_03.nc for time_bnds [0,6]
ocn_middle_0002_01_01_09.nc for time_bnds [6,12]
ocn_middle_0002_01_01_15.nc for time_bnds [12,18]

"ocn_begin%4yr%2mo%2dy%2hr", 6, "hours", 1, "hours", "time", 6, "hours", "2 1 1 0 0 0" 0, "", "begin"
generates the files:
ocn_begin_0002_01_01_00.nc for time_bnds [0,6]
ocn_begin_0002_01_01_06.nc for time_bnds [6,12]
ocn_begin_0002_01_01_12.nc for time_bnds [12,18]

"ocn_end%4yr%2mo%2dy%2hr", 6, "hours", 1, "hours", "time", 6, "hours", "2 1 1 0 0 0" 0, "", "end"
generates the files:
ocn_end_0002_01_01_06.nc for time_bnds [0,6]
ocn_end_0002_01_01_12.nc for time_bnds [6,12]
ocn_end_0002_01_01_18.nc for time_bnds [12,18]

"ocn_default%4yr%2mo%2dy%2hr", 6, "hours", 1, "hours", "time", 6, "hours", "2 1 1 0 0 0"
generates the files:
ocn_default_0002_01_01_03.nc for time_bnds [0,6]
ocn_default_0002_01_01_09.nc for time_bnds [6,12]
ocn_default_0002_01_01_15.nc for time_bnds [12,18]

AM4.1 runs with intel/debug on gaea still work.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes