added fits_add_fit to fits_save_fit

changes in fits_save_fit:
- decreased _n_up to 998 as this was the maximal number fv could display on HERCULES
- added mjdref field to saved structure
- changed fits_read_key in _fsf_add_string to fits_read_key_int_frac if key = MJDREF
- added if/else statement to select if key is Double or String in _fsf_add_string
- read in TSTART, TSTOP, and MJDREF with _fsf_add_string function (Keys not present in 1st extension of XMM data)
- instead of using the "target" field from get_datainfo, "target" is now determined by the "OBJECT" key of the PHA file
- _fsf_add_string starts at HDU 0 to look for the necessary keys
- added fits_add_fit

changes in merge_struct_arrays:
- added 'reshape = dim' qualifier, to reshape multidimensional arrays to the original shape, with dim being increased by the number of structures merged