Missing features

[simonrw]

This tracks features that are missing from the library that should be included. This is not a complete list, but mostly features that I use, or have been requested.

• control mutability of files
• read a fits image by start and number of elements
• read a fits image by square region
• read a fits column
• read a fits column range
• write a fits image by start and number of elements
• write a fits image by square region
• write a fits column
• write a fits column range
• read/write variable length columns
• read/write string columns
• null values
• add/remove columns to a table
• ascii tables - do we want to support this?!
  • for now, we don't support ascii tables
• copy hdu (#44)
• get image data type
• resize fits image (#31)
• support extended filename syntax (66bcd87)
• provide (safe) access to the underlying fitsio_sys::fitsfile object (#32)
• delete hdu (#45)
• get hdu name (#45)
• get number of hdus (#45)
• hdu iterator (#46)
• support images which are not 2D
• change image data type issue