♻️ Use .nc data files instead of .ascii

[jsnel]

Proposed refactoring

In general we wish to steer away from using the (legacy) .ascii format which is a bit tricky for new users to create as well as less precise and less space efficient. One way to do that is to use the 'right' format in public facing examples, such as in this repository.*

_{* Fully acknowledging that these 'examples' were originally not intended as examples but more as 'development validation' test code, while also acknowledging they are now often used as a 'starting' point and any example code here may get copy-pasta'd around.}

And there probably should be a little guide in the (pyglotaran) documentation how to prepare a xarray dataset from 'scratch', from a spreadsheet, from a .csv file, or something else people might commonly have their data formatted in.

Benefits

• Gives user the incentive to use .nc files with their own code

Additional context