tridiag
Solve tridiagonal system of equations,
` y=tridiag(dm1,d,dp1,rhs)`
 Inputs `dm1` The subdiagonal below the main diagonal. `d` The main diagonal. `dp1` The subdiagonal above the main diagonal. `rhs` The right hand side of the equation. Outputs `y` The solution of the tridiagonal system of equations.

Description
The function solves the diagoal system of equations:
`     d[1]x[1]+dp1[1]*x[2] =rhs[1];`
`    dm1[1]x[1]+ d[2]*x[2]+dp2[2]*x[3] =rhs[2];`
`     dm1[2]*x[2]+ d[3]*x[3]+dp1[3]*x[4] =rhs[3];`
`    ...`
`    ...`
No pivoting is used, and so the matrix defined by the inputs must be non-singular and numerically well-conditioned for the results to be reliable.

Example
```>>a=[2 -1 0 0;-1 2 -1 0;0,-1,2 -1 ;0 0 -1 2]
>>a
2         -1          0          0
-1          2         -1          0
0         -1          2         -1
0          0         -1          2

>>rhs=[2 3 4 5]'
>>a\rhs
6.0000
10.0000
11.0000
8.0000

>>dm1=-ones(4,1),d=ones(4,1)*2,dp1=dm1
>>tridiag(dm1,d,dp1,rhs)
6.0000
10.0000
11.0000
8.0000

```