BUG: snow_age state variable should have explicit units (seconds)

[jhamman]

The snow_age state variable is currently stored in units of "timesteps". I'm proposing that we change the units of this variable in vic_run and in the state files for all drivers to something that does not depend on the model timestep (e.g. seconds). The current approach fails if the user changes the timestep when restarting the model.

xrefs: https://github.com/UW-Hydro/VIC/pull/402, https://github.com/UW-Hydro/tonic/pull/43#discussion_r57246936