Add new block_trust_region MEX

This MEX solves nonlinear systems of equations using a trust region algorithm.
The problem is subdivided in smaller problems by doing a block
triangularisation of the Jacobian at the guess value, using the
Dulmage-Mendelsohn algorithm.

The interface of the MEX is simply:

  [x, info] = block_trust_region(f, guess_value);

Where f is either a function handle or a string designating a function.
f must take one argument (the evaluation point), and return either one or two
arguments (the residuals and, optionally, the Jacobian).

On success, info=0; on failure, info=1.