2-D approximation coefficients

The input vector `C`

and bookkeeping matrix `S`

contain all the information about the 2-D signal decomposition.

Let `NMAX = size(S,1)-2`

; then ```
C = [A(NMAX) H(NMAX) V(NMAX)
D(NMAX) … H(1) V(1) D(1)]
```

where `A`

,
`H`

, `V`

, and `D`

are vectors. If
`N = NMAX`

, then a simple extraction is done; otherwise,
`appcoef2`

computes iteratively the approximation coefficients
using the inverse wavelet transform.