Initial fix for issue 2555
Small modifications to enable keyword retrieval
Pass requested keywords as a string.
Allow multiple --key arguments
--keyword instead of --key
--keyword instead of --key
Adds support for the fitsheader --table feature
Adds support for the fitsheader --table feature
Simplified code and improved docstrings
Adds a test for the fitsheader --table feature
improved the usage string and tests
improved the usage strings and tests
Refactored the fitsheader script and improved its tests.
Undo the changes to header.py
Added code formatting suggestions by @embray
Merge pull request #2588 from barentsen/fixes-2555

Add option to `fitsheader` to extract a specific keyword