Use unit metadata, especially time

[rafaqz]

We should be able to automatically derive the step size of time grids and similar from the units. It may also be possible to do this by subtracting them, but may also not be exact.

It's also useful for series where layers are single slices of time in separate files, but may contain metadata on the duration of the slice. Currently this needs to be added manually as the time dim of the series is not guaranteed to actually be an evenly-spaced range.

This may require some conversion of units to Unitful.Quantity or DateTime

[rafaqz]

NetCDF time periods are now handled correctly. Although half the time they aren't included when they should be.