Perpendicular Laplacian inversion using FFT and Tridiagonal solver.
Equation solved is \(d*\nabla^2_\perp x + (1./c)\nabla_perp c\cdot\nabla_\perp x + a x = b \), where \(x\) and \(x\) are perpendicular (X-Z) or 3D fields, and \(a\) and d are 2D fields. If d is not supplied then it is 1
Flags control the boundary conditions (see header file)
Copyright 2010 B.D.Dudson, S.Farley, M.V.Umansky, X.Q.Xu
Contact: Ben Dudson, email@example.com
This file is part of BOUT++.
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