Draft PR - Add testing for dtype of glider df

[celestinelindarto]

Hi @ocefpaf,

I've had a go at forking the gliderpy repository and tried adding a testing method to check if the dtype of the glider df meets our expectations. This draft pull request contains changes in the test_fetchers.py file.

Let me know what you think of the changes.

Thank you.

Note: There are several errors with the checks - I'll try looking into this and give you an update.

[ocefpaf]

@celestinelindarto some dtype checking is nice but that should be done by trying to conform the fetched data and not at the test phase. We want the fetch code to error out if the columns are not in the dtype we expect.

We can work on this and more when the GSoC results are in. I'm closing this for now to avoid confusion.