Handle field counts that aren't a regular grid

[will-moore]

Hopefully Fixes #168.

To test:

• Open a Well that has a number of fields that doesn't fit in a grid using the preview build: https://deploy-preview-169--vizarr.netlify.app/

[will-moore]

Tested successfully at https://github.com/hms-dbmi/vizarr/issues/168#issuecomment-1542222271