Add more geometry terms to outputs

[theo-brown]

While TORAX doesn't compute or need these, it would be useful if they appeared in the output file, as they are required for computing some derived quantities:

• <1/R>
• <B>

For example, with #1363, I need to be able to convert from <j.B> into <j_ϕ> and then into JETTO's "JZ" definition. This conversion requires <1/R> and <B>.

[theo-brown]

I think I managed to find a way of doing #1363 without these terms!

[jcitrin]

OK good to know. Regardless we can add these geometry terms. For example, <1/R> is needed for other things, as is e.g. <B^2> and <1/B^2>, so might as well throw in there as well.

[theo-brown]

Copying my comment from the duplicate issue (I'd forgotten that I'd already created one, sorry for duplication):

In #1371 I've found that I'm having to implement <1/R> numerically. This could be added to geometry.Geometry.

Rather than doing this in #1371, I've chosen to leave a TODO, for the following reason.

In the IMAS schema, <1/R> is labelled as gm9. However, the other geometry terms do not have the same naming in IMAS and TORAX - presumably because TORAX naming convention follows RAPTOR?

Example: <1/R^2> is gm1 in IMAS but g3 in TORAX.

I'll make a PR to add this term, but I just wanted a bit of guidance on naming, and whether I should refactor all the "g" terms to match IMAS names.

[theo-brown]

My <1/R> implementation:

torax/torax/_src/sources/electron_cyclotron_source.py

Lines 119 to 121 in 4e1d0b9

	# Numerical trick to avoid division by zero: dρ/dA = dρ²/dA * 1/(2ρ)
	darea_drho2 = jnp.diff(geo.area_face) / jnp.diff(geo.rho_face**2)
	fsa_1_R = geo.F * geo.g3 * darea_drho2 / (jnp.pi * geo.B_0)

[jcitrin]

In the IMAS schema, <1/R> is labelled as gm9. However, the other geometry terms do not have the same naming in IMAS and TORAX - presumably because TORAX naming convention follows RAPTOR?
Example: <1/R^2> is gm1 in IMAS but g3 in TORAX.
I'll make a PR to add this term, but I just wanted a bit of guidance on naming, and whether I should refactor all the "g" terms to match IMAS names.

The TORAX names are consistent with ASTRA + RAPTOR conventions. There is no need for the g# names to match the IMAS gm# names. They are different conventions and that's fine. The IMAS<-->TORAX convertors handle this. Once we have IMAS outputs of TORAX and the users work with this, then the internal names will be practically invisible to users anyway.