improve the computation of the Jacobian matrix

the Jacobian B*A depends only on choice, thus pre-calculate B*A for
each choice j, then duplicate it to each i according to the corresponding
choice.
fix bug to pick u s11 if it is not scaled
fix bug when covariance matrix approaches singular

add mychol.m to decompose singular positive semidefinite matrix
and use it in ProbitProb.m to replace chol() function, which raises errors
due to rounding errors.

other minor modifications