mtlb_uint8 Matlab uint8 emulation function

## Description

Matlab and Scilab `uint8` behave differently :

- Beyong the main interval bounds, Scilab's `uint8()` wraps, while Matlab's `mtlb_uint8()` saturates.
- For non integer values, Scilab's `uint8()` truncates the fractional part, while Matlab's `mtlb_uint8()` rounds to the nearest integer.

x | uint8(x) | mtlb_uint8(x)
--- | --- | ---
%nan | 0 | 0
-%inf | 0 | 0
-2 | 254 | 0
-1 | 255 | 0
0 | 0 | 0
2 | 2 | 2
10.2 | 10 | 10
10.5 | 10 | 11
10.51 | 10 | 11
... | ... | ...
255 | 255 | 255
256 | 0 | 255
257 | 1 | 255
%inf | 255 | 255

`mtlb_uint8` has not to be used for hand coded functions.

The function `mtlb_uint8(A)` is used by `mfile2sci` to replace `uint8(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.