Expand grdfill's aliases to include coltypes (f)

[weiji14]

Description of the desired feature

Continuing on from #1276, there are some parameters in upstream GMT at https://docs.generic-mapping-tools.org/6.2/grdfill.html which are still missing in PyGMT at https://www.pygmt.org/v0.4.0/api/generated/pygmt.grdfill.html. Your task is to fill them in!

Current state :memo:

https://github.com/GenericMappingTools/pygmt/blob/d90b3fc889b53633deab6b4ab77612ac7a247c1b/pygmt/src/grdfill.py#L21-L22

https://github.com/GenericMappingTools/pygmt/blob/d90b3fc889b53633deab6b4ab77612ac7a247c1b/pygmt/src/grdfill.py#L54-L55

Desired state :sparkles:

You'll need to change two parts of the grdfill.py module like so:

    R="region", 
    V="verbose", 
    f="coltypes",

    {R}
    {V}
    {f}

How to make the change :monocle_face:

1. First, read up our contributing guidelines at https://www.pygmt.org/v0.4.0/contributing.html#editing-the-documentation
2. Leave a comment below, stating that you will tackle this issue. We will assign you to this issue and you can then start working on it.
3. To edit the grdfill module's docstring, go to https://github.com/GenericMappingTools/pygmt/edit/master/pygmt/src/grdfill.py and add in the new docstring!
4. Submit a pull request, and be sure to fill it up with as much detail as possible.

Tip: See the Pull Request at #1343 for an example of how a common alias can be added.

Stretch goals :rocket:

Suggest a good long alias name for the grdfill -N parameter, and add it to the Pull Request!

:partying_face: Good luck, and let us know if you need any help :)

Are you willing to help implement and maintain this feature? Happy to review someone's Pull Request on it.