Squashed commit of the following:

commit eb236762
Author: Conor Anderson <conor@conr.ca>
Date:   Wed May 27 07:36:12 2020 -0400

    Add note to README

commit ecc5817b
Author: Conor Anderson <conor@conr.ca>
Date:   Thu May 21 08:43:01 2020 -0400

    Better error detection in cropncdf

commit 4fab42b5
Author: Conor Anderson <conor@conr.ca>
Date:   Thu May 21 07:48:23 2020 -0400

    Fix year in copyright

commit 70931d3a
Author: Conor Anderson <conor@conr.ca>
Date:   Thu May 21 07:46:10 2020 -0400

    Fix some option labels

commit 0c2d9218
Author: Conor Anderson <conor@conr.ca>
Date:   Thu May 21 07:36:11 2020 -0400

    Update .gitignore

commit a1ce26b6
Author: Conor Anderson <conor@conr.ca>
Date:   Thu May 21 07:28:50 2020 -0400

    User-facing updates for this implementation

commit 35461e38
Author: Conor Anderson <conor@conr.ca>
Date:   Tue May 19 07:21:09 2020 -0400

    Allow cropping multiple files