NetCDF files require eta?

[amoodie]

It was brought to my attention that eta may be a required variable for deltametrics to allow a netcdf file to be read. This should not be the case. There may be a few locations where we check for eta (mostly in relation to stratigraphy), but these should optionally support other variables.

There is no reason eta should be a required variable.

[elbeejay]

For testing purposes our sample_data.landsat() and the first return of the sample_data.savi2020() sample datasets are netCDF files which do not have the eta variable.