Low-storage Schemes and Option `solveorderconditions`

[ranocha]

Currently, the option solveorderconditions cannot be used for low-storage schemes:

>> rk=rk_opt(5, 2, '3Sstar', 'acc', 'num_starting_points', 2, 'solveorderconditions', 1, 'algorithm', 'interior-point')
Error using  * 
Incorrect dimensions for matrix multiplication. Check that the number of columns in the
first matrix matches the number of rows in the second matrix. To perform elementwise
multiplication, use '.*'.

Error in order_conditions (line 16)
lin_tau = Aeq*x'-beq';

Error in rk_opt>@(x)order_conditions(x,class,s,p,Aeq,beq) (line 113)
        x(i,:) = fsolve(@(x) order_conditions(x,class,s,p,Aeq,beq), x(i,:));

Error in fsolve (line 242)
            fuser = feval(funfcn{3},x,varargin{:});

Error in rk_opt (line 113)
        x(i,:) = fsolve(@(x) order_conditions(x,class,s,p,Aeq,beq), x(i,:));

Caused by:
    Failure in initial objective function evaluation. FSOLVE cannot continue.

While the error message clearly points to the point of failure and suggests the workaround to not use solveorderconditions for low-storage methods, it is still nice to document this problem (done via this issue) and probably also to fix it if needed.