Conceal use of Eigen

[fsimonis]

Please describe the problem you are trying to solve.

At the time of writing, Eigen is responsible for around 40% of our overall compile time of preCICE.
Primary offender are the uses of Eigen::VectorXd all around the code-base. It is not possible to use anything related to meshes, vertices or data, without (transitively) including Eigen/Core.

A common recommendation to solve this problem is to use Eigen only inside compute-heavy compilation units and use something else for storing data. In practise, this means that data storage is handled by std::vector and then interfaced with using Eigen::Map.

Describe the solution you propose.

Remove the use of Eigen from all central headers of preCICE (related to mesh and data):

• Use span<const double> where ranges of doubles need to be accepted or exposed.
• Use std::vector<double> to store data-values and gradients.

Move Eigen accessor functions to separate headers that can be included if necessary:

• Instead of Eigen::VectorXd mesh::Vertex::getCoords(), implement Eigen::Vector toCoords(const mesh::Vertex&); and Eigen::Map<const Eigen::VectorXd> asCoords(const meah::Vertex&) in a separate header.
• Move utility functions such as Eigen::VectorXd getCenter(const Edge&) to a utility header.

Replace unnecessary use of Eigen in Tests:

• Use mesh.createVertex({1, 2, 3}) instead of mesh.createVertex(Eigen::Vertex3d{1, 2, 3})
• Use std::vector<double> expectedValues{1,2,3,4} instead of Eigen::VectorXd expected(4); expected << 1, 2, 3, 4;
• If necessary, write custom comparators which internally use Eigen.

Describe alternatives you've considered

Precompiled headers could help with this, but this needs some serious testing.

Additional context

#1973