Add multicomponent Freundlich and Sips isotherms

[ronald-jaepel]

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

I've rebased the branch onto master and added two minor changes to documentation and tests.

Status and ToDos:

• [x] Multicomponent Freundlich:
  • [x] Add Flux and Jacobian Implementation
  • [x] Add tests
  • [x] Complete tests
  • [x] Add documentation
• [ ] Multicomponent Sips:
  • [x] Add Flux and Jacobian Implementation
  • [x] Add tests
  • [ ] Complete tests
  • [x] Add documentation

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

For the Sips isotherm, the tests fail if any cp == 0.

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[ronald-jaepel]

Closing. Upon consideration I'll split this into two PRs. One for each Isotherm.