document requirements for TIFF files

It seems, many different kinds of TIFF are generated at instruments worldwide, and not all of them can be read by BornAgain.

• In the webdocs, describe requirements for TFF files to be read, and recommend conversion tools. Also invite users to send us samples of not yet supported TIFF files.
• In the GUI code, upon failure to read TIFF file, print message with link to the docs.