pygmt.grdgradient: Migrate the 'azimuth'/'radiance' parameters to the new alias system

[seisman]

Just regular migration. No other changes.

[yvonnefroehlich]

If we change "azim" to "azimuth" we need to update the following docstrings.

Suggested change

	Azimuthal direction for a directional derivative; *azim* is the
	angle in the x,y plane measured in degrees positive clockwise from
	north (the positive y-direction) toward east (the positive x-direction). The
	Azimuthal direction for a directional derivative; *azimuth* is the
	angle in the x,y plane measured in degrees positive clockwise from
	north (the positive y-direction) toward east (the positive x-direction). The

Unfortuantely I can only suggest changes for the first few lines.

        negative of the directional derivative,
        :math:`-(\frac{{dz}}{{dx}}\sin(\mbox{{azimuth}}) + \
        \frac{{dz}}{{dy}}\cos(\mbox{{azimuth}}))`, is found; negation yields
        positive values when the slope of :math:`z(x,y)` is downhill in the
        *azimuth* direction, the correct sense for shading the illumination of an
        image by a light source above the x,y plane shining from the *azimuth*
        direction. Optionally, supply two azimuths, *azimuth*/*azimuth2*, in which
        case the gradients in each of these directions are calculated and the
        one larger in magnitude is retained; this is useful for illuminating
        data with two directions of lineated structures, e.g., *0*/*270*
        illuminates from the north (top) and west (left).  Finally, if *azimuth*
        is a file it must be a grid of the same domain, spacing and
        registration as *grid* that will update the azimuth at each output
        node when computing the directional derivatives.

[seisman]

I've changed it to azim. Will work on the docstrings in future PRs instead.