Implemented the pipe_control.error_analysis.covariance_matrix() function.
This follows from http://thread.gmane.org/gmane.science.nmr.relax.scm/23526/focus=7096. It will be
used by a new error_analysis.covariance_matrix user function. And it calls the specific API methods
model_loop(), covariance_matrix(), and set_error() and the relax library
lib.statistics.multifit_covar() function do to most of the work.
