Expand grdgradient's aliases to include normalize (N)

[weiji14]

Description of the desired feature

Continuing on from #1269, there are some parameters in upstream GMT at https://docs.generic-mapping-tools.org/6.2/grdgradient.html which are still missing in PyGMT at https://www.pygmt.org/v0.4.0/api/generated/pygmt.grdgradient.html. Your task is to fill them in! Note that this issue may be slightly more difficult, so please look at another simpler issue like #1371 (unless you're comfortable with a challenge)!

Current state 📝

pygmt/pygmt/src/grdgradient.py

Lines 23 to 24 in d90b3fc

	G="outgrid",
	R="region",

pygmt/pygmt/src/grdgradient.py

Lines 91 to 92 in d90b3fc

	they will be ignored.)
	{R}

Desired state ✨

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

    G="outgrid",
    N="normalize", 
    R="region", 

        they will be ignored.) 
    normalize : str
        ...
    {R}

where the ... is to be copied from https://github.com/GenericMappingTools/gmt/edit/master/doc/rst/source/grdgradient.rst, and modified slightly so that short params like -N are renamed to their long form like normalize

How to make the change 🧐

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/grdgradient.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 or #1190 for an example of how a common alias can be added.

Stretch goals 🚀

Find another missing long alias for grdgradient such as the lambert -E parameter, and add it to the Pull Request!

🥳 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.

[weiji14]

Was going to mark this as good-first-issue but looks like you're working on this in #1515 @willschlitzer. Could you suggest another module that hasn't been fully aliased yet so I could change this issue (and save myself from having to open a new one) 😃