Explicitly check for "EN" at end of .ori files

[jdbuhler]

MEGAlib .ori files end with the tag "EN" on a line by itself. This tag is removed when the file is read in. Recently, we discovered that some .ori files in cosipy were missing that terminal EN; those files were fixed, but we still need to properly detect the missing EN.

This patch chooses to reject files without the terminal EN, rather than attempt to work around them, because trying to "do the right thing" would need to handle all sorts of cases. For example, there might be multiple lines of garbage at the end of the file.

This PR is intended to supersede #516.

Since I was editing this file anyway, I also included a one-line fix for issue #527 with a suitable test case.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.74%. Comparing base (0d7d6d6) to head (3c220e3).

Files with missing lines	Patch %	Lines
cosipy/spacecraftfile/spacecraft_file.py	85.71%	1 Missing ⚠️

Files with missing lines	Coverage Δ
cosipy/spacecraftfile/spacecraft_file.py	84.23% <85.71%> (+0.50%)	⬆️

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