Allow the LDLT_factor class to be used when computing the trace of an (inverse) quadratic form.
Update matrix includes to include quad_forms.
Make use of trace_inv_quad_form_ldlt in multi_gp_log.
Make use of quad_form and trace_inv_quad_form_ldlt in multi_normal_log.
Add a matrix_normal_prec distribution.
Expose matrix_normal_prec_log and quadratic forms to the model parser.
Add unit test for quad_form and quad_form_sym.
Add unit test for trace_quad_form.