Semi-modular indexing for assignment, and other improvements

• Command-line help for T and F.
• Some errata corrected in help
• Behaviour of Xl (clamp) with 2 inputs changed
• Octave compatibility: Yd (blkdiag) now acccepts char inputs, like Matlab
• Corrected Zt (strrep). If first input is char or a cell array, numerical second and third inputs are converted to char.
• Avoid paging output in Octave, so that it honours pauses etc. Thanks, Suever!
• In Z@ (randperm), if first input is char it is interpreted as population (not as size)
• Octave compatibility for strcat: a single string as input removes trailing space
• Semi-modular indexing for assignment