Create a separate repository for demo data

[juseg]

Not all hyoga functionality can be demonstrated with the Zenodo data. Besides, these data are already post-processed and thus not ideal to show what happens when opening a raw PISM (or other models) file. Finally, the file is very large, using a lot of resources on read the docs and for anyone who would like to use the demo data. Create and use a separate repository containing:

• PISM demo boot file
• PISM demo series file
• PISM demo gridded file
• PISM demo interp file

When this is done new examples will be added to

• Plot bedrock uplift
• Plot interpolated data