Improve 'Isotope' window readability

[Liam-Twomey]

I suggest adding column headings to the data table in the isotopes.m GUI, to improve ease of use. I have implemented this in my fork (see below) and am ready to PR, but I wanted to be respectful of the contribution rule against PRs which do not respond to an existing issue.

I think this falls under "interface or implementation improvements of an existing function", but I understand if you think it falls under "tiny code refactors and 'beauty' edits".

[stestoll]

This definitely improves usability, so it's a welcome addition - thank you!

Please submit PR, making sure it's up-to-date relative to the main branch.