Improve portability w/ hdw file download

[Wtristen]

Scope

Modified pydarn.utils.superdar_radars.py 'get_hdw_files' to improve code portability between OS platforms.
This does not change hdw files themselves or affect any other part of pydarn. The solution replaces external command-line dependencies curl and unzip in get_hdw_files with native python modules urllib.request and zipfile.

issue:
Resolves issue #402 and anticipated portability bugs with use of curl and string path (replaced with os.path)

Approval

Number of approvals: number of approvals before one can merge the code

Test

matplotlib version:
Note testers: please indicate what version of matplotlib you are using

provide a list or code to test

Code to test was uploaded under test/test_hdw.py. To reproduce the issue, use a platform that does not include unzip or curl or one that does not support '/' paths (like old windows I believe); and ensure your hdw directory is empty.

*Reminder, please check your code is:

• copyrighted (if applicable)
• Appropriate disclaimer added
• Modification line filled in (if applicable)*

[Wtristen]

Sorry if I've goofed something up! This is the first pull-request I've ever attempted and I'm generally new to version control.

[carleyjmartin]

Installed and imported no issue, installed hdw files on installation no issue.
Removed hdw files and used get_hdw_files() with no issues also.

For now I would remove the test_hdw file, or you can look into pytest and redo it to look similar to the other test files so it works with pytest. It can also be added as a test within test_utils too if that's easier?

Comments and suggestions are just keeping style consistent and tidying. I use flake8 to check pep8 styling if, which you can install with pip and run like flake8 file.py and it will give you places to check.

Great PR!

[Wtristen]

I believe I've implemented the changes recommended by @carleyjmartin on notation and I've removed the test_hdw.py file that I had previously added in this branch.

[carleyjmartin]

Same tests as before, all good.

I should probably find a windows PC to test this on...