Fetch models WIP

[ChetGoerzen]

Fixes #30

This pull request adds the ability to download the AK135-f, IASP91, MEAN, PEM-A, PEM-C, PEM-O, MC35, STW105, and TNA/SNA earth models.

The following things have been completed

• Functions to download all of the above models
• Examples have been written for all of the above models

The following things are still incomplete

• Some tests do not pass due to exact equalities being used for testing calculations with floats
• Some of the code does not conform to PEP8 guidelines
• make format and make check have not been run
• Docstrings require proofreading
• New functions have not been added to doc/api/index.rst

Reminders

• [x] Run make format and make check to make sure the code follows the style guide.
• [x] Add tests for new features or tests that would have caught the bug that you're fixing.
• [x] Add new public functions/methods/classes to doc/api/index.rst.
• [x] Write detailed docstrings for all functions/methods.
• [x] If adding new functionality, add an example to the docstring, gallery, and/or tutorials.

[welcome[bot]]

💖 Thanks for opening this pull request! 💖

Please make sure you read our Contributing Guide and abide by our Code of Conduct.

A few things to keep in mind:

• Remember to run make format to make sure your code follows our style guide.
• If you need help writing tests, take a look at the existing ones for inspiration. If you don't know where to start, let us know and we'll walk you through it.
• All new features should be documented. It helps to write the docstrings for your functions/classes before writing the code. This will help you think about your code design and results in better code.
• No matter what, we are really grateful that you put in the effort to do this! ⭐

[santisoler]

Nice job @ChetGoerzen! :+1:

Let us know when you want us to review the code.

[ChetGoerzen]

I believe that I have completed everything. However, I have been unable to find the expanded form of many of the earth model names. Some models like PREM have the expanded form "Preliminary Reference Earth Model" while some models like IASP91 do not seem to have an expanded form. Even when I read the original paper describing the earth model I am unable to find the expanded form of the name. Will this be alright if some models don't have the expanded form? If this is acceptable should I open another pull request without the WIP tag?

[ChetGoerzen]

It looks like some of the checks were unsuccessful. I will resolve these errors before opening another pull request.

[leouieda]

Hi @ChetGoerzen sorry for letting this PR hang. I'll have a look as soon as I can at what is hapening with the Python 3.7 build.

[ChetGoerzen]

Hi @leouieda, thanks for looking into this. I am not familiar with Azure pipelines so I have been unable to troubleshoot the problem myself.