phase: models: use signed char instead of char

[rwk-unil]

Because the C/C++ standard does not specify the signedness of the char type the results will be compiler/machine dependent this can lead to unexpected behaviour.

Use signed char instead of char to avoid these cases.

References :

• https://stackoverflow.com/questions/2054939/is-char-signed-or-unsigned-by-default
• https://gcc.gnu.org/onlinedocs/gcc-4.2.2/gcc/C-Dialect-Options.html
• https://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf

Fixes : https://github.com/odelaneau/GLIMPSE/issues/242