Removed time unit from OptionsGlobalProperties.

Removed time unit from OptionsGlobalProperties.

Replaced the shared time unit value with a separate TimeUnitManager
class that provides basic functionality for time unit conversions, can
add an option for setting the time unit, and can scale time options
according to a time unit. Control flow should be easier to follow now
that the time options don't implicitly interact.

Part of issue #839.

Change-Id: Ib89459fae744a17e7d7c98a4d7e878b0c2dcf1ab