Feature/stokes drag

@toba will need to study a new type of particle, described by second order equations. These are particles for which the equations are the following:

    \begin{aligned}
        \frac{d x_i}{dt} &= p_i \\
        \frac{d p_i}{dt} &= -\mu(p_i - u_i)
    \end{aligned}

where x_i is the position of the particle (3 components), p_i is the momentum of the particle (3 components, basically velocity), and u_i is the fluid velocity at the particle location (3 components).

As far as the implementation is concerned, these particles have a 6-dimensional state vector just like the particles with orientation, but the equations are fundamentally different.

See also issue #33 (closed) that I originally opened to discuss this.

The branch I want to merge here is my attempt to expand the API to handle this type of particle.

Before merging I would like

1. @bbramas to have a quick look to see whether my changes don't mess up the API.
2. @toba to design a sanity check that we can then implement and use to validate the implementation.

Sidenote: I was somewhat overeager to better organize testing for TurTLE, therefore this branch also contains a new CMake testing configuration. This somewhat addresses issue #22 (closed).