pygmt.grdfilter: Support Pythonic arguments for the distance parameter

[seisman]

grdfilter's -D option is the distance flag indicating the meaning of the grid and the width. It can accept one of p or 0-5, but distance=4 is very confusing. This PR makes it take more readable arguments.

The long names come from

https://github.com/GenericMappingTools/gmt/blob/328f373b348747abcb2279456778e7f60567eb49/src/grdfilter.c#L149-L155

#define GRDFILTER_XY_PIXEL		-1
#define GRDFILTER_XY_CARTESIAN		0
#define GRDFILTER_GEO_CARTESIAN		1
#define GRDFILTER_GEO_FLATEARTH1	2
#define GRDFILTER_GEO_FLATEARTH2	3
#define GRDFILTER_GEO_SPHERICAL		4
#define GRDFILTER_GEO_MERCATOR		5

Preview: https://pygmt-dev--4405.org.readthedocs.build/en/4405/api/generated/pygmt.grdfilter.html