mtlb_int16 Matlab int16 emulation function

## Description

Matlab and Scilab `int16` behave differently :

- Beyong the main interval bounds, Scilab's `int16()` wraps, while Matlab's `mtlb_int16()` saturates.
- For non integer values, Scilab's `int16()` truncates the fractional part, while Matlab's `mtlb_int16()` rounds to the nearest integer.

x | int16(x) | mtlb_int16(x)
--- | --- | ---
%nan | 0 | 0
-%inf | -32768 | -32768
-32770 | +32766 | -32768
-32769 | +32767 | -32768
-32768 | -32768 | -32768
-32767 | -32767 | -32767
-80.7 | -80 | -81
-80.5 | -80 | -81
-80.4 | -80 | -80
10.2 | 10 | 10
10.5 | 10 | 11
10.51 | 10 | 11
32767 | +32767 | 32767
32768 | -32768 | 32767
32769 | -32767 | 32767
32770 | -32766 | 32767
%inf | +32767 | 32767

`mtlb_int16` has not to be used for hand coded functions.

The function `mtlb_int16(A)` is used by `mfile2sci` to replace `int16(A)` when it is not possible to know what is the input while porting Matlab code to Scilab. This function then determines the correct semantic at run time.