Dynamic allocations

I'd like to integrate this library in a Xenomai setup but I'm afraid of dynamic allocations. I see some [push_back](https://github.com/GiulioRomualdi/osqp-eigen/blob/cd18d43fb0698438f7e9e9fc38d0640e498787b3/include/OptimizerSolver.tpp#L243) in the code.
I was wondering If we could take advantage of the `reserve` methods for `std::vector<>` and [eigen sparse matrices](https://eigen.tuxfamily.org/dox/group__SparseQuickRefPage.html) toa void any memory allocation between solver updates.