convenience function to compute frequency from wavelength

[tylerflex]

would be nice to provide a shortcut to compute frequency from wavelength as we use frequency generally internally and lots of users work with wavelengths. In our examples and internally, we often have freq = td.C_0 / wavelength, but it would be nice for users to do this conversion without needing to know the formula.

Add a new utility function td.frequency(wavelength) that computes the frequency from a wavelength in um.

Perhaps we can define it in tidy3d/constants.py.

Then we should probably replace all instances of td.C_0 / wavelength with this function in both the source code and also the docs.