Bad NO_SHEAR_MODES ns2d.strat

There are 2 parameters NO_SHEAR_MODES:

params.NO_SHEAR_MODES
params.oper.NO_SHEAR_MODES

See https://foss.heptapod.net/fluiddyn/fluidsim/blob/branch/default/fluidsim/solvers/ns2d/strat/solver.py#L72

The second one should be fine but the first one seems to be buggy.

https://foss.heptapod.net/fluiddyn/fluidsim/blob/branch/default/fluidsim/solvers/ns2d/strat/time_stepping.py#L188

The block is useless and key_fft[0, :] = 0 assumes too much about how the arrays in Fourier space are organized!