Frequency conversion utility function

[AndrewSykes1]

Adds feature described #1746

[tylerflex]

Thanks @AndrewSykes1 ! We appreciate the contribution. After some internal discussion (which we should have updated in the original issue..). We think it would be better to have this function named wvl_to_freq so it's more explicit and there's less chance of confusion as to what it does.

Also, adding a changelog item under added would be useful to alert users to this (eg. `

- Convenience function for converting from wavelength (um) to frequency (Hz) using `td.wvl_to_freq(wvl)`

If you want, I can pick up this PR and make the changes, or you could modify it if you have time. Up to you.

Thanks again!

[AndrewSykes1]

Sounds good, I'll have time to make the changes so should be no problem. Thanks for the correction!