Better type checking in `eigensolve_isotropic_media`

Currently, if the wavelength parameter within eigensolve_isotropic_media is passed as a float, rather than a jax array, then it throws an obscure error (not directly pointing to the wavelength parameter itself).

It would be good to either

• explicitly allow for float values and do the necessary conversions under the hood.
• assert a type up front and throw an error describing why this needs to be the case.