imwrite: implement WriteMode option. * imwrite.m: document the new option WriteMode and possibility to write multipage images by passing a 4 dimensional matrix. * private/core_imwrite.m: perform input check for the quality option and the new writemode. Set defaults here and not on __magick_write__(). Give warning about the fact that writing of indexed images is not properly implemented. Change calling to ind2rgb() since it has been there and we no longer need workaround. Remove the different calls to __magick_read__() since we now have a single way to do it. Remove conversion of image types since we want to save what was actually given to us. * __magick_read__.cc (read_file): split from __magick_read__() into a separate function so it can be used by __magick_write__() when appending images to an existing file. (jpg_settings): remove function. It only checks for the quality option, which is now done by core_imwrite(). Plus, other formats support this option so it was moved into __magick_write__(). We should have functions for each option rather than per file format. (encode_map): comment whole function since it is never used and is unfinished work to implement writing of actual indexed images. (write_file): new function from part of previous write_image(). It is now the other side of read_file(). (write_image): remove function. Moved into __magick_write__(), the only function calling it. The part of writing moved into write_file(). (__magick_write__): removed most of input check which should be done by imwrite(). Removed all extra usage types. Options must be passed on a non-optional struct. Implement the Append option.