Add a TimeDeltaScaleSystem

**Problem Description**

The `CalendarScaleSystem` expects times in floating point seconds since the unix Epoch. This causes problems when the times are essentially deltas since some starting point, rather than absolute times (see #545 for example).

In this case it would be good to have a weeks/days/hours/minutes/seconds scale system (or and perhaps some others for years, and months to use independently) which expects either timedelta64 or floating point seconds.