Better warning messages for RF simulations

[yuanshen-flexcompute]

With the newly introduced check for whether a Tidy3D simulation contains RF functionality, I wonder if we can introduce a different set of conditions for triggering warning messages. In the long run, it would be good to have a different set of warnings and triggers for optical vs RF sims.

That said, in the short term, there is one warning message that is very prevalent:

WARNING: Structure at structures[0] was detected as being less than half of a central wavelength from a PML on side y-min. To avoid inaccurate results or divergence, please increase gap between any structures and PML or fully extend structure through the pml.

Can we find a way to disable this message for simulations flagged as RF in the short term? Reasoning:

• A lot of RF simulations are sub-wavelength and it is normal for structures to be fairly close to the PML.
• One warning message is generated for each structure and each boundary, leading to lots of spam and potentially obscuring other more important warnings.

If we don't want to disable this warning, perhaps consolidating all such warnings into one message would help.

Thanks

[tylerflex]

Maybe we can wait for the pydantic v2 to wrap up, because validators are handled differently there. It would be nice if we had access to the initlalized simulation object so we could simply check the RF flag and call this validator if it's not present. @yaugenst-flex