Implement for matfaust.poly.expm_multiply the same extension (grouping several...

Implement for matfaust.poly.expm_multiply the same extension (grouping several series of coeffs -- option group_coeffs) as the one already integrated in pyfaust.poly (c5496442) which was modified here to be auto used (group_coeffs=True) when tradeoff='time' (as matfaust does).

Refactor also the C++ code behind (uniform signatures polyGroupCoeffs* when using this extension)