Merge pull request #14041 from mantidproject/13994_loadfits_tidying_fixes_improvements

Everything looks good, the parsing of comment headers has been adapted to the standard including a comment in the doxygen string which is very helpful. I downloaded some telescope data (9 MB) and copied it 100 times, the loading time went down by approximately 50% (I guess it depends on data and computer to some degree). Tests still pass on Jenkins.