Use structures for input/output arguments in `pd_gmres`

[jhabriel]

Things got out of hand...

We got to a point where we have too many input arguments and too many output arguments.

After #28 is tackled, we will have to re-refactor the signature of the function using structures. We can have a call of the type

[x, extraOutputArgs] = pd_gmres(A, b, extraInputArgs, varargin);

where extraOutputArgs and extraInputArgs are respectively the extra output and input arguments.

[jhabriel]

Another alternative could be:

[x, flag, relresvec, mvec, time] = pd_gmres(A, b, mInitial, extraInputs, varargin);