add example of gp code

gp/example.gp

+\r omf
+\\ Quadratic form of level 19
+q = [2, 0, 0, 1, 1; 0, 2, 0, 1, 0; 0, 0, 2, 0, 1; 1, 1, 0, 2, 0; 1, 0, 1, 0, 6];
+\\ We initialize the orthogonal modular forms for q
+omf = omf_init(q);
+\\ We initialize the information to compute T2 for k = 3, j = 2 (a = 1, b = 1)
+t2 = omf_tpabd_init(omf, 2, 1, 1);
+\\ We compute the invariant space for d = 1
+inv = omf_invariantsabd(omf, 1, 1, 1);
+\\ We compute the Hecke operator for p = 2, d = 1 (we have to multiply by p^a)
+T2 = omf_tpnd(t2[1], 1, inv, t2[2])*2;