Stop closing files opened by the caller

[jackriccomini]

Fixes #164

Note: this was a very quick edit on a Friday afternoon before I log off for the week.

I have only run through a test in the case that the file is passed in (fh_is_managed = True), not in the other case (fh_is_managed = False).

Please do make changes if needed

[addisonElliott]

Yep, this looks good. One thought is to rename fh_is_managed to something like detached_data or data_detached since a fh is only opened when the data is detached from the header. Just a nit.

Can you add some tests for this? You could probably update the existing tests to verify they're properly closing or not closing the fh.

[jackriccomini]

Fair point about the variable name. Whilst making that change, I realised introducing a new variable is unneccesary and I could simplify by checking data_filename is not None instead.

I have amended my commit to include that change and a unit test