Implement Particle refilling

Solves the following issue(s):

Closes #272 (closed)

Model-specific changes:

• The function boundary_transfer has been modified.

So far, it only transfers the particle to the opposite poloidal angle \theta but now it also considers the toroidal angle \phi in order to put the particle at the same magnetic flux surface.

• New function particle_refilling has been implemented.

When particle reaches outer boundary, then it does not consider as a lost particle but it is re-inserted at the same magnetic flux.

\theta_\text{refill} = - \theta_\text{loss}
\\
\phi_\text{refill} = \phi_\text{loss} - 2 q(r_\text{loss}) \theta_\text{loss}

*Currently only valid in case of poloidal geometry with AdhocTorus equilibrium.