Add satnum setter

[ckuethe]

Fixes #97. I'm sure I missed some finer points, but I ginned up a little set of tests and everything seems to work:

• roundtrip unmodified TLEs
• setting satnum as an int: 123456
• setting satnum as a string: "123456"
• setting satnum as an alpha5: "C3456"
• rejecting out-of-range satnums
• rejecting badly formatted alpha5: "WALDO", "O0000"
• Exporting TLEs with alpha5
• Loading TLEs with alpha5

[ckuethe]

Here are some test cases ... https://gist.github.com/ckuethe/eb37e8a99d9f7fd5842d2e54212e62ad

[brandon-rhodes]

Thanks for trying to put this together! But I wound up resolving the issue with much less code, as explained here:

https://github.com/brandon-rhodes/python-sgp4/issues/97#issuecomment-1525482029

Hopefully that will let users write to the satellite number, but without burdening the module with too much extra code.