n_quad is now mandatory in spline_space_1d, not optional.

n_quad is now mandatory in spline_space_1d, not optional. The default is set to n_quad=6, docstrings have been adapted. Projectors PI that take callables have been added to the new classes projectors_tensor. PI and PI_mat have been compared in the projector tests. interp_mapping has been replaced by PI_0. name change in projectors_global_1d: pi_0_v2 --> pi_0_mat, pi_1_v2 --> pi_1_mat. Parameters in test_ files adapted.