grdclip: Facilitate mulplite reclassification

I copy the question / feature request from [Circular (slope-aspect) color scale "bar"](https://forum.generic-mapping-tools.org/t/circular-slope-aspect-color-scale-bar/1117/1):

Is there a simpler way to do this?

```
gmt grdclip %SLOPE_GRID% -G%CUT% -Sb3/10 -Si3/12/20 -Si12/22/30 -Sa22/40
gmt grdclip %ASPECT_GRID% -G%CUT2% -Sb22.5/1 -Si22.5/67.5/2 -Si67.5/112.5/3 -Si112.5/157.5/4 -Si157.5/202.5/5 -Si202.5/247.5/6 -Si247.5/292.5/7 -Si292.5/337.5/8 -Sa337.5/
```

To be clear. Instead of the above, to write something like this (which NOT exist, I just take the idea of -C and -T from makecpt). The “-” in the begging and in the end are for -Sb and -Sa (or MIN and MAX). Hope to be clear.

```
gmt grdclip %SLOPE_GRID% -G%CUT% -C10,20,30,40 -T-,3,12,22,-
gmt grdclip %ASPECT_GRID% -G%CUT2% -C1,2,3,4,5,6,7,8 -T-,22.5,67.5,112.5,157.5,202.5,247.5,292.5,337.5,-
```

**Are you willing to help implement and maintain this feature?** Yes