Describe b0 as the on-axis R B_phi over the axis major radius at phi = 0

[CharlesCNorton]

Change - the VmecWOut.b0 docstring says what the field holds: the on-axis R B_phi (rbtor0) divided by the major radius of the magnetic axis at phi = 0, which is the field strength on the axis only for a planar circular axis.

Cause - it read "Magnetic field magnitude on the magnetic axis". b0 is fpsi0 / r_00 in output_quantities.cc, with fpsi0 the half-grid bvco extrapolated to the axis and r_00 the axis R at the first grid point, as eqfor.f computes it; on a non-planar axis neither is the field there.

Evidence - pyQSC's r2 section 5.4 quasi-helical configuration (B0 = 1, nfp 4, axis R from 0.847 to 1.190) solved at ns 201: |B| on the axis is 1.0000 within 6e-4 at every toroidal angle, rbtor0 is 1.2009, which is B0 times the axis length over 2 pi (1.2009), the axis R at phi = 0 is 1.1895, and b0 is 1.0096.

Scope - docstring only.

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