sim.output.phys_fields.plot not good enough
`phys_fields.plot` raises too many errors (in particular in 3D). ```python from fluidsim import load_state_phys_file as load; sim=load() # this works sim.output.phys_fields.plot() # but this raises a ValueError sim.output.phys_fields.plot(time=200) ``` ``` ValueError: key "rotz" not in state.keys_state_phys (['vx', 'vy', 'vz']). The quantity cannot be computed because time is not None. ``` - [ ] When needed, a `state` can be added to contain data for a particular time and compute variables (we already have the operator). Similarly, ```python from fluidsim import load_sim_for_plot as load; sim=load() sim.output.phys_fields.plot() ``` raises ``` ValueError: key "rotz" not in state.keys_state_phys (['vx', 'vy', 'vz']). "rotz" in sim.state.keys_computable but sim.params.ONLY_COARSE_OPER is True ``` (and `sim.output.phys_fields.plot("vx")` works). - [x] by default, a variable that we can get should be plotted. - [ ] also check `sim.output.phys_fields.animate`
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