generalise contractions

[martinjrobins]

At the moment only 2d to 1d contractions are supported, should generalise this to include:

• nd to n-1d
• nd to md where n > m
• other accumulations than +, e.g. max/min

[martinjrobins]

perhaps the syntax for other accumulations could be:

u_i__max__ { a_ij }

or

u_i { (0)_max_: y = a_ij }

u_i { _max_: a_ij }

or

u_i { (0)[max]: y = a_ij }

u_i { [max]: a_ij }