spatial_means_regions module does not launch when state_phys_t* files are not present.

At least one state_phys_t* file is required for loading the spatial_means_regions module at initialization.

Python 3.9.5 | packaged by conda-forge | (default, Jun 19 2021, 00:32:32) 
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IPython 7.25.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: ls
info_solver.xml  params_simul.xml  spatial_means_regions/  spatial_means.txt  spatiotemporal/  spect_energy_budg.h5  spectra1d.h5  spectra3d.h5  spectra_kzkh.h5  state_phys_t00140.004.nc  stdout.txt

In [2]: from fluidsim import load_sim_for_plot as load
   ...: sim=load()
*************************************
Program fluidsim
sim:                <class 'fluidsim_core.extend_simul.extend_simul_class.<locals>.NewSimul'>
sim.output:         <class 'fluidsim.solvers.ns3d.strat.output.Output'>
sim.oper:           <class 'fluidsim.operators.operators3d.OperatorsPseudoSpectral3D'>
sim.state:          <class 'fluidsim.solvers.ns3d.strat.state.StateNS3DStrat'>
sim.time_stepping:  <class 'fluidsim.solvers.ns3d.time_stepping.TimeSteppingPseudoSpectralNS3D'>
sim.init_fields:    <class 'fluidsim.solvers.ns3d.init_fields.InitFieldsNS3D'>
sim.forcing:        <class 'fluidsim.solvers.ns3d.forcing.ForcingNS3D'>

solver ns3d.strat, RK4 and sequential,
type fft: fluidfft.fft3d.with_pyfftw
nx =      4 ; ny =      4 ; nz =      4
Lx = 7.5 ; Ly = 7.5 ; Lz = 1.5
path_run =
/.fsnet/project/watu/2020/20MILESTONE/Bardant/ownCloud/2021-stage-bardant-antoine/data/ns3d.strat_600x600x120_V7.5x7.5x1.5_N2_Lf6.5_U0.2_D0.5_2021-06-08_14-07-38
init_fields.type: constant

Initialization outputs:
sim.output.phys_fields:       <class 'fluidsim.base.output.phys_fields3d.PhysFieldsBase3D'>
sim.output.spatial_means:     <class 'fluidsim.solvers.ns3d.strat.output.spatial_means.SpatialMeansNS3DStrat'>
sim.output.spatial_means_regions:<class 'fluidsim.extend_simul.spatial_means_regions_milestone.SpatialMeansRegions'>
sim.output.spatiotemporal_spectra:<class 'fluidsim.solvers.ns3d.output.spatiotemporal_spectra.SpatioTemporalSpectraNS3D'>
sim.output.spectra:           <class 'fluidsim.solvers.ns3d.strat.output.spectra.SpectraNS3DStrat'>
sim.output.spect_energy_budg: <class 'fluidsim.solvers.ns3d.strat.output.spect_energy_budget.SpectralEnergyBudgetNS3DStrat'>
sim.output.temporal_spectra:  <class 'fluidsim.base.output.temporal_spectra.TemporalSpectra3D'>

Memory usage at the end of init. (equiv. seq.): 188.74609375 Mo
Size of state_spect (equiv. seq.): 0.003072 Mo

In [3]: rm state_phys_t00140.004.nc

In [4]: sim=load()
*************************************
Program fluidsim
sim:                <class 'fluidsim.solvers.ns3d.strat.solver.Simul'>
sim.output:         <class 'fluidsim.solvers.ns3d.strat.output.Output'>
sim.oper:           <class 'fluidsim.operators.operators3d.OperatorsPseudoSpectral3D'>
sim.state:          <class 'fluidsim.solvers.ns3d.strat.state.StateNS3DStrat'>
sim.time_stepping:  <class 'fluidsim.solvers.ns3d.time_stepping.TimeSteppingPseudoSpectralNS3D'>
sim.init_fields:    <class 'fluidsim.solvers.ns3d.init_fields.InitFieldsNS3D'>
sim.forcing:        <class 'fluidsim.solvers.ns3d.forcing.ForcingNS3D'>

solver ns3d.strat, RK4 and sequential,
type fft: fluidfft.fft3d.with_pyfftw
nx =      4 ; ny =      4 ; nz =      4
Lx = 7.5 ; Ly = 7.5 ; Lz = 1.5
path_run =
/.fsnet/project/watu/2020/20MILESTONE/Bardant/ownCloud/2021-stage-bardant-antoine/data/ns3d.strat_600x600x120_V7.5x7.5x1.5_N2_Lf6.5_U0.2_D0.5_2021-06-08_14-07-38
init_fields.type: constant

Initialization outputs:
sim.output.phys_fields:       <class 'fluidsim.base.output.phys_fields3d.PhysFieldsBase3D'>
sim.output.spatial_means:     <class 'fluidsim.solvers.ns3d.strat.output.spatial_means.SpatialMeansNS3DStrat'>
sim.output.spatiotemporal_spectra:<class 'fluidsim.solvers.ns3d.output.spatiotemporal_spectra.SpatioTemporalSpectraNS3D'>
sim.output.spectra:           <class 'fluidsim.solvers.ns3d.strat.output.spectra.SpectraNS3DStrat'>
sim.output.spect_energy_budg: <class 'fluidsim.solvers.ns3d.strat.output.spect_energy_budget.SpectralEnergyBudgetNS3DStrat'>
sim.output.temporal_spectra:  <class 'fluidsim.base.output.temporal_spectra.TemporalSpectra3D'>

Memory usage at the end of init. (equiv. seq.): 219.25390625 Mo
Size of state_spect (equiv. seq.): 0.003072 Mo