compute lead. monomials of GB only => speedups for dimension and Hilbert series computations

We should modify the interface with msolve to add an option to groebner_basis so that msolve computes only the leading monomials of the Groebner basis. 
This is usually faster and would then speed-up computations of the Hilbert series and/or of the dimension of an ideal when the base field is the field of rational numbers. 