Allow more boundary parametrizations than double Fourier series

[dpanici]

• Add method to surfaces to get linear operator going from that surface DOF to double Fourier series
• make BoundaryRSelfConsistency use this linear operator in its compute method
• then this should plumb well through perturbations and proximal projection
• Figure out how to then modify proximal projection etc for this change
• do we want to keep Rb_lmn, change its definition, or add new variables?

for e.g. using the Garabedian or Henneberg representations, or using some custom basis based on PCA modes like the analysis in Giuliani's work

[dpanici]

• RealSpaceRZSurface where R Z points in space are the DOFs and whenever needs to compute something, will fit with Rb_mn Zb_mn (for use with say the proximal stuff internally or bdryRZconsistency)
• Spline representation @IssraAli

[dpanici]

• PCA modes (can use LinearObjectiveFromUser, just need to form the matrix projecting Rb, Zb onto the PCA basis)