Using std::vector or boost::multi_array would make ufc easier to use

UFC has several 2D arrays implemented as double**, both as members and function arguments.

Using a flat double* would make the interface easier to use in many cases.

Using a flat std::vector<double> would be even easier to use, at no performance penalty and no significant dependency.

Using a boost::multi_array<double,2> would retain the easy to generate and read A[i][j] syntax, at the cost of a boost dependency, and eventual performance penalties (could be zero).