Merge two FITS headers ATL_MGFTS
1 - Concatenation. Store the contents of FC1 in the returned FitsChan, and then append the contents of FC2 to the end of the returned FitsChan. No checks are made for multiple occurences of the same keyword.
2 - Union (with priority given to FC2): For every header in FC1, see if FC2 contains the same keyword. If it does not, copy the FC1 header to the returned FitsChan. Then append the contents of FC2 to the end of the returned FitsChan.
3 - Overlap: For every header in FC1, see if FC2 contains the same keyword. If it does, and if the keyword value is the same in both FitsChans, copy the FC1 header to the returned FitsChan.
4 - Union (with priority given to FC1): Copy FC1 to the output, then for every header in FC2 append it to the end of the returned FitsChan unless the card already exists. This is similar to method 2 except that cards from FC2 are dropped instead of cards from FC1.
The contents of FC1 and FC2 are unchanged on exit.
For METHOD 3 (overlap), floating point values are compared by formatting into a string (using the accuracy specified by the FitsDigits attributes of the two supplied FitsChans) and then comparing the formatted strings for exact equality.
Method 4 exists to allow a new header to be appended whilst retaining the primary order of the cards from the first header.