feature/rapid equilibrium reactions for CSTR

[AntoniaBerger]

Fixes #20.
This PR will add the feature to set reactions into a rapid equilibrium in liquid phase for the CSTR

Workflow:

• Add a function that calculates the mass matrix with respect to conserved moieties.
• Update residualImpl
• Update consitentInitalisation
• Update Jacobian
• Update derivative Jacobian
• Run tests and make sure my changes don't break existing functionality.
• Add test, that ensures the functionality of the new feature.
• Update the documentation.
• Make Eigen libary obligatory for CADET

More information about the algorithm are available under the forum post about reactions in rapid equilibrium

[jbreue16]

if you write the keyword to close the issue right in front of the corresponding issue, it will automatically be closed when the PR is merged. I.e. writing "fixes" instead of "resolves therefore" in the PR description .. maybe "resolves" also works

[jbreue16]

I think its time to make Eigen obligatory for CADET.
Currently, Eigen is only installed when the ENABLE_DG flag is set to ON (which is the default)
So when you are using Eigen in another submodule, the build will fail when ENABLE_DG is OFF

We can either make Eigen obligatory for CADET or you also guard the rapid-equilibrium reaction code with compile time flags checking for Eigen.
You can see this e.g. under /src/libcadet/CMakeLists.txt where the DG units are only included if the flag is ON

This is only relevant for the merge into master, so for now you can also just set this as the last todo in your todo list