Add rountrip tests

[ofgulban]

I have added an initial roundtrip test. @nausikaa8 , would you have time to have a look and see if sth is missing (see here)? If it looks good to you, I am going to proceed adding other file formats to the roundtrip tests.

[nausikaa8]

Excellent, @ofgulban! (Tiny remark: pytest is imported twice.)

Since your code is concise and generic, do you think it would be possible to use function handles to have one single roundtrip function for all file formats? As arguments for this function the file format read and write functions would be passed, for example:

def test_header_roundtrip(<read_format_func_name()>, <write_format_func_name()>):

Or would it be more clear to have explicit an roundtrip test function for each file format?