Modify the type of seconds since midnight

[quasi-mod]

• closes #xxx
• tests passed
• new tests added
• science test figures
• ran uncrustify prior to final commit
• ReleaseNotes entry

This PR changes the type of seconds_since_midnight and the explicit cast of curr_date_time. the previous implementation casted curr_date_time, which is a time_t value (64bit value), to unsigned int and took the mod. However, casting 64bit value to 32bit value (uint) will result in loosing upper 32 bits, and thus will result in the value wrapping around (btw, the exact year time_t value will wrap around in unsigned int is year 2106).
To prevent this, curr_date_time should be casted to u64 type.

Thanks in advance for your consideration.

[jhamman]

Thank you for your contribution. This all looks fine to me. @tbohn / @yixinmao / @bartnijssen - care to take a peak?

[bartnijssen]

We should probably use snprintf() rather than sprintf() in vic_log.c anyway.