MAINT - Use ``create_dual_point`` in group and multitask lasso

[Badr-MOUFAD]

celer/celer/group_fast.pyx

Lines 237 to 238 in 4230db1

	tmp = 1. / n_samples
	fscal(&n_samples, &tmp, &theta[0], &inc)

celer/celer/multitask_fast.pyx

Lines 332 to 333 in 4230db1

	tmp = 1. / n_samples
	fscal(&n_obs, &tmp, &theta[0, 0], &inc)