Fix matfaust.Faust.factors when the factor is a single (precision) sparse matrix.

>> factors(matfaust.rand(10, 10, 'class', 'single'), 1)
Error using mexFaustRealFloat
newMxGetData: the mex matrix must be float as the ptr is.

Error in matfaust.Faust/call_mex (line 4795)
                                                [varargout{1:nargout}] =
                                                mexFaustRealFloat(func_name,
                                                F.matrix.objectHandle, varargin{:});

Error in matfaust.Faust/factors (line 2460)
                                factors{j} = call_mex(F, 'factors', i);