*INTERFACE:*

subroutine tridiagonal(N,fi,lt,value)

A linear equation with tridiagonal matrix structure is solved here. The main
diagonal is stored on `bu`, the upper diagonal on `au`, and the
lower diagonal on `cu`, the right hand side is stored on `du`.
The method used here is the simplified Gauss elimination, also called
Thomas algorithm.

*USES:*

IMPLICIT NONE

integer, intent(in) :: N,fi,lt

REALTYPE :: value(0:N)

Original author(s): Hans Burchard & Karsten Bolding

Karsten Bolding 2012-12-28