Default of nmax is probably not sensible
The nmax input (https://gyrokinetics.gitlab.io/gs2/page/namelists/index.html#le_grids_knobs-nmax) is used to control the maximum number of points in integration intervals for parts of the velocity grids. This controls the maximum order of polynomial used in producing the quadrature grids. By default nmax = 500, meaning that the maximum polynomial order is only capped to 500 – meaning for high resolution cases we end up with high order polynomials. Typically such polynomials are not well behaved for orders above ~20 (as highly oscillatory etc.) and we can find numerical instability in very high resolution simulations which can be avoided by reducing nmax to lower values (this also significantly improves the initialisation time of high theta resolution cases with new_trap_int = T).
We should probably default to a lower nmax value (say 15?) to avoid hitting these known issues whilst still allowing the user to change this if they really want to.
