Relax constraints relating naky to ny and ntheta0 to nx in box mode.

Currently ny and nx have to exactly match the de-aliasing requirements - we set ny/nx and determine the naky and ntheta0 values we’re allowed to have. This typically leads to a conflict between choosing ny/nx values with low prime factors and naky/ntheta0 values with plenty of factors to provide many sweetspots. By decoupling the choice of ny and naky / nx and ntheta0 we are able to separately choose ny/nx/naky/ntheta0 to try to satisfy both performance requirements.

With the code in this PR we now just require ny and nx to be at least as large as required for de-aliasing in the nonlinear term. We still need to ensure that de-aliasing conditions are met, so this code aborts if the choice of grids does not satisfy this.