feat: Basic field strength equalities

[jstoobysmith]

Adding some lemmas which rewrite the field strength in a more familiar form. These will eventually be used to remove all the Tensor stuff from EM.

[zhikaip]

I think the docstring should be fixed, whether the mathematical properties can be generalised can be left for future work

[zhikaip]

does this work for a general SpaceTime d → Lorentz.Vector d ⊗[ℝ] Lorentz.Vector d type?

To me it seems a bit uneconomic to have these results proven specifically for one physical object only where presumably it's just mathematical manipulations, but maybe there are difficulties I'm not aware of

[zhikaip]

maybe this is a more general discussion and out of scope for this particular PR, if that's the case I'm happy to approve it first

[jstoobysmith]

I added a TODO item about this. I tried to do this but it needs a bit more API I think.

[zhikaip]

Suggested change

	We define the field strength tensor `F_μν` in terms of the derivative of the

do you mind fixing the indices throughout?

[jstoobysmith]

Done.

[zhikaip]

thanks for the changes, approved.

I've been thinking whether FieldStrength should get its own type, but maybe that's a much larger design decision to be made. Either way I think separating the maths from the physics as much as possible (generalising the former, then applying) is probably the way to go throughout physlib