Add more IMAS outputs

[theo-brown]

Minimal set required for coupling to TORAX (see google-deepmind/torax#1470):

• vacuum_toroidal_field.r0
• vacuum_toroidal_field.b0
• time_slice.global_quantities.ip
• time_slice.global_quantities.magnetic_axis.z
• time_slice.profiles_1d.psi , poloidal flux
• time_slice.profiles_1d.phi, toroidal flux
• time_slice.profiles_1d.r_inboard, radial coordinate (major radius) on inboard side of magnetic axis
• time_slice.profiles_1d.r_outboard, radial coordinate (major radius) on outboard side of magnetic axis
• time_slice.profiles_1d.f $= RB_\phi$
• time_slice.profiles_1d.volume, volume enclosed by flux surface
• time_slice.profiles_1d.gm7 $= \langle |\nabla \rho_\mathrm{tor} |\rangle$
• time_slice.profiles_1d.gm3 $= \langle |\nabla\rho_\mathrm{tor} |^2 \rangle$
• time_slice.profiles_1d.gm2 $= \langle |\nabla\rho_\mathrm{tor} |^2 / R^2 \rangle$
• time_slice.profiles_1d.gm1 $= \langle 1/R^2 \rangle$
• time_slice.profiles_1d.gm9 $= \langle 1/R \rangle$
• time_slice.profiles_1d.j_phi, toroidal plasma current density
• time_slice.profiles_1d.rho_tor_norm, toroidal flux coordinate normalised to LCFS
• time_slice.profiles_1d.triangularity_upper see imas geometry definitions
• time_slice.profiles_1d.triangularity_lower see imas geometry definitions
• time_slice.profiles_1d.elongation see imas geometry definitions

See TORAX EQDSK convertor for how some of these are calculated.