Add Multicomponent Freundlich LDF isotherm.

[ronald-jaepel]

This PR continues work by @sleweke to add the multicomponent Freundlich isotherm to CADET-Core.

I've rebased the branch onto master.

Status and ToDos:

• [x] Add Flux and Jacobian Implementation
• [x] Add tests
• [x] Complete tests
• [x] Add documentation

The Freundlich isotherm works in all tests configurations I threw at it (both components c_p > 1, cp == 0 and cp < 0).