Add -C[cpt] arg to grdmix

[PaulWessel]

This will let grdmix build an image directly from a grid and a CPT (and optionally handle intensity grid as well). Somehow left out in version 1. Includes a new test script to make sure this works.

[github-actions]

Summary of changed images

This is an auto-generated report of images that have changed on the DVC remote

Status	Path
deleted	test/baseline/grdmix/grdmix.ps
modified	test/baseline/grdmix/

Image diff(s)

Added images

Modified images

Path	Old	New

Report last updated at commit 28c89e9

[seisman]

@PaulWessel It's likely that you forgot to run dvc push when working on this PR.

You need to run dvc push to see if any local changes will be pushed.

[PaulWessel]

Ok, so do this in master since already merged?

[seisman]

Yes

[PaulWessel]

Done. Got

dvc push
  0% Checking cache in 'https://dagshub.com/GenericMappingTools/gmt.dvc/files/md5'|                                                                                                                                          |0/? [00:00<?,    ?files/s]Exception ignored in: <finalize object at 0x10f801560; dead>
Traceback (most recent call last):
  File "weakref.py", line 591, in __call__
  File "fsspec/implementations/http.py", line 121, in close_session
  File "fsspec/asyn.py", line 80, in sync
NotImplementedError: Calling sync() from within a running loop
Aiohttp retry client was not closed
2 files pushed