Merge GinacExVf and GinacMatrix

[vincentchabannes]

These classes are very similar and can be merged in order to simplify the dev/support of symbolic expr. We can also support this kinds of expr (all identical) :

auto a1 = expr( "3*u+v:u:v");
auto a2 = expr( "{3*u+v}:u:v");
auto a3 = expr<1,1>( "3*u+v:u:v");
auto a4 = expr<1,1>( "{3*u+v}:u:v");