All grid datasets should carry unyt strings as attributes.

[WillJRoper]

In an extension to #530 every dataset in a grid file should contain units which we can then read and attach to a Grid. As it stands we assume erg / s / Hz for all luminosities but we could find ourselves in a different situation. If we carry the units this is explicit and can be more flexible in the future.

[WillJRoper]

The only missing ingredient here is units on spectra and lines in grids from cloudy. This is being implemented in the grids repo.