v0.10.4

• Equilibrium.map_coordinates is now differentiable.
• Removes method Equilibrium.compute_flux_coordinates as it is now redundant with the
  more general Equilibrium.map_coordinates.
• Allows certain objectives to target FourierRZToroidalSurface objects as well as
  Equilibrium objects, such as MeanCurvature, PrincipalCurvature, and Volume.
• Allow optimizations where the only object being optimized is not an Equilibrium
  object e.g. optimizing only a FourierRZToroidalSurface object to have a certain
  Volume.
• Many functions from desc.plotting now also work for plotting quantities from
  Curve and Surface classes.
• Adds method FourierRZToroidalSurface.constant_offset_surface which creates
  a surface with a specified constant offset from the base surface.
• Adds method FourierRZToroidalSurface.from_values to create a surface by fitting
  (R,phi,Z) points, along with a user-defined poloidal angle theta which sets the poloidal
  angle for the created surface
• Adds new objective LinearObjectiveFromUser for custom linear constraints.
• elongation is now computed as a function of zeta rather than a single global scalar.
• Adds beta_vol and betaxis to VMEC output.
• Reorder steps in solve_continuation_automatic to avoid finite pressure tokamak with
  zero current.
• Fix error in lambda o(rho) constraint for near axis behavior.
• Fix bug when optimizing with only a single constraint.
• Fix some bugs causing NaN in reverse mode AD for some objectives.
• Fix incompatible array shapes when user supplies initial guess for lagrange multipliers
  for augmented lagrangian optimizers.
• Fix a bug caused when optimizing multiple objects at the same time and the order of
  the objects gets mixed up.

Full Changelog: v0.10.3...v0.10.4