Parson's v2: Extension to grids and groupings

[smmercuri]

• The parsons block is now extended to handle multiple columns (for grouping questions) and grid layouts (for matching and other grid-style questions) through the use of columns and rows block header parameters. The original proof Parsons version of v4.5.0 is obtained by specifying neither columns nor rows.
• Update the Sortable.css styles for grid styling, which is separate to the proof Parsons styling.
• Add matchlib.mac Maxima library containing helper and required functions for grouping and grid questions using the parsons block. These help to translate between Maxima arrays and relevant JSON structure, as well as some useful assessment functions.
• CSS bug fix for proof assessment display
• Documentation for Matching problems with examples.

[sangwinc]

Thanks @smmercuri, looks great.