Improve performance of get_step_size

[billsacks]

From @billsacks on August 22, 2017 23:48

get_step_size in clm_time_manager is called from many places in CLM's run loop. It may be worth improving its efficiency. This could easily be done by storing the result as a module-level variable in initialization, and then returning this stored value whenever get_step_size is called - rather than querying the ESMF object every time this routine is called.

Copied from original issue: NCAR/CLM#9