AR_estimate
===========
```{function}  AR_estimate( t, val, freq, < nu, mu, expand_factor >)
```
Full definition:
```{code-block} matlab
[val_ar t_ar f_val_ar  EC] = AR_estimate( t, val, freq, < nu, mu, expand_factor >)
```

apply autoregressive signal model to improve dft results

* t     : time vector
* val   : time domain values
* freq  : frequency vector for dft

optional

* nu    : AR order (default 40)
* mu    : number of timesteps to train the model (default 3*nu)
* expand_factor : increase signal length by this factor (default 5)

return values:

* val_ar:   AR estimated time signal
* t_ar:     time vector
* f_val_ar: FD transformed AR estimated signal
* EC:       error code
            0 --> no error
            1 --> input error: t and val mismatch
            2 --> input error: mu has to be larger than 2*nu
            3 --> input error: expand_factor has to be larger than 1
            10 --> AR error: signal is to short for AR estimate --> decrease AR order
            11 --> AR error: estimated signal appears to be unstable --> use a different mu



-----------------------
Author: Thorsten Liebig, 2011

See also ReadUI, DFT_time2freq