Fix/flux structure

[schmoelder]

The ordering of the flux was not consistent with other solution types. For a consistent interface, this PR reorders the flux structure.

Since flux is hardly used by anyone, I believe a breaking change is justifiable.

@sleweke: Is it enough to change the solution layout or do we also need to restructure the contents of the state vector?

[schmoelder]

I was wrong.

bulk: 'nTime', ('nAxialCells',) ('nRadialCells' / 'nPorts',) 'nComp'
particle_liquid: 'nTime', ('nParTypes',) ('nAxialCells',) ('nRadialCells' / 'nPorts',) ('nParShells',) 'nComp'
particle_solid: 'nTime', ('nParTypes',) ('nAxialCells',) ('nRadialCells' / 'nPorts',) ('nParShells',) 'nComp', 'nBound'
flux: 'nTime', ('nParTypes',) ('nAxialCells',) ('nRadialCells' / 'nPorts',) 'nComp'