Custom window for phenometrics

[Aariq]

Adds a year_start and year_end argument, with some input validation, to npn_get_data_by_year() and (eventually) functions that call npn_get_data_by_year()

Still to do:

• double check that everything in Custom window for phenometrics #79 is dealt with
• update NEWS.md with new arguments
• update examples
• add tests

[Aariq]

This is ready for review I think. I removed the code that added start_date and end_date columns because I realized it isn't appropriate for all functions that call npn_get_data() (e.g. npn_download_status_data() results already have a column for observation date). I'll open an issue describing where specifically this may be needed as it might be more complicated to handle this.

[Aariq]

This is ready for review I think. I removed the code that added start_date and end_date columns because I realized it isn't appropriate for all functions that call npn_get_data() (e.g. npn_download_status_data() results already have a column for observation date). I'll open an issue describing where specifically this may be needed as it might be more complicated to handle this.