Fixed issue with stoneholmes fit for small noise data.

Negative log likelihood functions now switch to using variable precision arithmetic (vpa) if exponential function becomes infinite. This occurs in case of very small noise (i.e., small standard deviation) and/or large lambda_u values or passage times. Was returning error and/or NaN.

Also improved initial estimate of lambda_u to use unbiased estimator of variance (std(x,0) instead of std(x,1)).

Signed-off-by: Andrew D. Horchler <adh9@case.edu>