Improve parameter interface

[jbreue16]

For version 6, we should improve our parameter interface and make it more consistent.
See also CADET-Specification #3

• Improve particle interface: group PAR_TYPE_xxx which holds all particle parameters like radius, pore diffusion, binding.
• Change COLUMN_DISPERSION to COLUMN_DISPERSION_AXIAL (2D models already have COLUMN_DISPERSION_RADIAL)
• make the velocity/cross section area/flow rate interface unique. Option1: remove velocity field, require cross_section/radius and add flow direction field.