Complete documentation of grdimage (#620)
Complete documentation of grdimage (#620)

* Document common aliases for grdimage

Add automatic docstrings for frame (B), cmap (C)
projection (J), region (R), timestamp (U), verbose (V),
and interpolation (n).
Also removed alias pen (W) that actually doesn't work.

* Alias dpi (E) for grdimage
* Copy grdimage long description from GMT manpage

Copied from https://docs.generic-mapping-tools.org/6.1/grdimage.html,
swapping out short command-line arguments for long aliases.

* Alias monochrome (M), no_clip (N) and nan_transparent (Q) for grdimage
* Alias img_out (A) and img_in (D) for grdimage
* Alias bit_color (G) for grdimage
* Remove mention of RGB grid inputs, following GMT 6.2.0 grdimage docs
* Document shading (I) for grdimage

Co-authored-by: Dongdong Tian <3eae6d0b95aef6af8a0d785fe9f127268703c43c@gmail.com>
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python