Updated documentation to reflect 'sel' usage: Changed isel to sel in docs/technotes/02_the_data_array.md and docs/usage/data_array.md
Introduced new 'sel' function: Added a more intuitive 'sel' function, which performs the same task as 'isel' and prepares for upcoming xrray syntax support.
Usage is analogous to .sel in xarray, not .isel!