Various improvements (see description)

• Null assignment (x(...)=[]) with a single index is now possible with function (
• Added functions to simplify indexing rows of columns of 2D arrays: Y), Z) for reference indexing; Y(, Z( for assignment indexing. The latter allow null assignment with a single index.
• Added function conv2(...,'valid') (Z+)
• cumsum, cumprod now allow char input
• Renamed functions "linearize array", "comma-separated list", gallery
• Octave compatibility for diff and mod with char input
• Octave compatibility with repelem (thanks, @beaker!)
• Fixed incorrect <strong>...</strong> tags in Octave