Reset cell and particle counter on load and skip cpu and level loops while loading if not required

[nvaytet]

With these changes:

osyris.Dataset(8, scale="au", path=path).load('sink') yields

Loaded: 0 cells, 0 particles.
Dataset: osyrisdata/starformation/output_00008: 128.00 B
Datagroup: sink 128.00 B
'id' Min: 1.0 Max: 2.0 [] (2,)
'msink' Min: 3.482e+32 Max: 3.485e+32 [g] (2,)
'position' Min: 3.383e+03 Max: 3.471e+03 [au] (2, 3)
'v' Min: 7.464e+04 Max: 7.577e+04 [cm / s] (2, 3)

(we skip both loops over CPUs and amr levels)

osyris.Dataset(8, scale="au", path=path).load('hydro') yields

Processing 12 files in osyrisdata/starformation/output_00008
 16% : read      65623 cells,          0 particles
 25% : read      90140 cells,          0 particles
 33% : read     118232 cells,          0 particles
 41% : read     147100 cells,          0 particles
 50% : read     170244 cells,          0 particles
 66% : read     235859 cells,          0 particles
 75% : read     260384 cells,          0 particles
 83% : read     288476 cells,          0 particles
 91% : read     312840 cells,          0 particles
Loaded: 340488 cells, 0 particles.
Dataset: osyrisdata/starformation/output_00008: 43.58 MB
Datagroup: hydro 43.58 MB
'density' Min: 2.376e-19 Max: 8.047e-12 [g / cm ** 3] (340488,)
'thermal_pressure' Min: 8.555e-11 Max: 0.057 [erg / cm ** 3] (340488,)
'internal_energy' Min: 1.379e+07 Max: 9.174e+15 [] (340488,)
'temperature' Min: 10.002 Max: 196.393 [K] (340488,)
'velocity' Min: 742.189 Max: 4.216e+05 [cm / s] (340488, 3)
'B_left' Min: 2.097e-05 Max: 0.164 [G] (340488, 3)
'B_right' Min: 2.097e-05 Max: 0.162 [G] (340488, 3)
'B_field' Min: 2.119e-05 Max: 0.157 [G] (340488, 3)

(we loop over CPUs and amr levels, but only have hydro variables in the output)

osyris.Dataset(8, scale="au", path=path).load('part') yields

Processing 12 files in osyrisdata/starformation/output_00008
 16% : read          0 cells,          0 particles
 25% : read          0 cells,        956 particles
 33% : read          0 cells,        956 particles
 41% : read          0 cells,        956 particles
 50% : read          0 cells,       2109 particles
 66% : read          0 cells,       2109 particles
 75% : read          0 cells,       3065 particles
 83% : read          0 cells,       3065 particles
 91% : read          0 cells,       4217 particles
Loaded: 0 cells, 4218 particles.
Dataset: osyrisdata/starformation/output_00008: 337.44 KB
Datagroup: part 337.44 KB
'mass' Min: 1.651e+29 Max: 1.652e+29 [g] (4218,)
'identity' Min: -2.0 Max: -1.0 [] (4218,)
'levelp' Min: 5.0 Max: 5.0 [] (4218,)
'birth_time' Min: 8.888e-05 Max: 8.892e-05 [] (4218,)
'position' Min: 3.354e+03 Max: 3.500e+03 [au] (4218, 3)
'velocity' Min: 7.464e+04 Max: 7.577e+04 [cm / s] (4218, 3)

(we skip the loop over amr levels and only load particles)