Extract path of selected year without creating and masking boolean column, and some rewording

See merge request mipdata/tutorials-and-use-cases!3

add the .copy() method and some rewording

See merge request mipdata/tutorials-and-use-cases!2

Numbering, rewording, and define to do's

See merge request mipdata/tutorials-and-use-cases!1

create two versions of summer day notebook, ome with era5 and cmip6 and one wiht only cmip6. There are still problems with finding the era5 coordniates