add direct formatting option to ImageKeyword struct with tests

[mileslucas]

For example usage, see the code in the test files ImCreate_cube.c and ImCreate_img.c.

Key points:

• specify F keyword type for formatted keywords
• Format as added in the place of the space-padding uint64 in the ImageKeyword struct. The number of bytes is, by default, set to 8 to match the original padding of the struct.
  • This should be sufficient for all format strings as they are %-16.16s, which is 8 bytes, but if we need breathing room, we can save the format strings without the % since it is redundant.
• Struct version incremented to 2.00 -> 2.01; this is not a breaking change, just a feature addition.

To fully implement these changes, code will need to be changed in milk (save_fits.c) and camstack.

[DasVinch]

Unfortunately, I have comments...

Changing type to 'F' is breaking
--> We need to hang on to native formats everywhere, and at the very last step of FITS-ifying we need to figure out that .format is not a null. Otherwise, all the other stuff loses capability to distinguish D, F and S.

Now, does the FITS writer know and enforce that a keyword is formattable?

• Having non null format field: yes but then everytime a keyword is reused, the format field MUST positively be cleared

• Add new formatters 'd' 'f' 's' for formatted int, float, string: that somewhat works, and lower/upper lets the FITS writer know whether the format is current or stale, and lets camstack set the formatters to lowercase based on the knowledge that there indeed is a valid formatter. HOWEVER, every single code that performs a kw.type check against 'D', 'S', 'F' must be changed to compare (0x7f & kw.type == kw.type.upper) instead.