Revise the function 'non_radial_pulsations' and add the function 'histogram2d_with_weights'

This commit revises the function 'non_radial_pulsations'. The
two most notable changes are:
1) The surface of the sphere is no longer sampled uniformly
by area but uniform in the phi-theta plane. This opens up the
possibility to easily interpolate the resulting velocity field
on a finer grid via a two-dimensional Taylor expansion, which
significantly improves the robustness and performance of the
function.
2) The effect of phase smearing has been incorporated, which
necessitates the additional input field 'delta_osc_phase'.

As a consequence of the changes to 'non_radial_pulsations',
the qualifiers 'N_interpolation' and 'no_negative_weights'
of the function 'histogram_with_weights' became obsolete
and have been removed. In turn, the two-dimensional analog
of the function, 'histogram2d_with_weights', has been added.