Including t_bound variable in a history file and resulting changes

[nmizukami]

Output time_bounds variable (datetime at time step endpoints). This is CESM history file standard, but it is useful information for standalone.

Changes made are:

• time variable in history file uses double precision
• new metadata for tbound added in meta_qDims and resulting change var_lookup.
• change timeVar from scalar to 2 element array.
• also TSEC array uses the same array index bound (1:2) as timeVar
• track time at bounds while history variable aggregation.
• moderate refactoring historyFile bound procedure - createNC
• fix comparison of simulation ending time and forcing ending time.

No answer change, but history file now has time_bounds with new dimension tbound (size=2)

Fix #387