fix pec freq arg

[dmarek-flex]

Small bug when trying to plot epsilon with coord_key not equals to centers and freq=None. Other media assume an infinite frequency but PEC results in TypeError at multiplication

eps_sample = sim.epsilon(box=box, coord_key="Ex")

[tylerflex]

Thanks @dmarek-flex, appreciate you submitting the fix along with the bug report.

Maybe let's try to merge this into develop just to get it out faster.
Also, can you add a line to the CHANGELOG.md under ### fixed?

Thanks!

[weiliangjin2021]

I wonder why not add the decorator @ensure_freq_in_range as in other materials?

[weiliangjin2021]

Maybe let's try to merge this into develop just to get it out faster. Also, can you add a line to the CHANGELOG.md under ### fixed?

If merged to develop, we'll need to merge develop to pre/2.7 since the development for RF is on pre/2.7 branch?

[tylerflex]

we could also just apply this commit to both branches potentially. Or it's ok if we keep merging this into pre/2.7 for simplicity.

[dmarek-flex]

I wonder why not add the decorator @ensure_freq_in_range as in other materials?

Yes, I can do that and it would fix it as well.