Detectors: New C++ implementation of detector particles

Initialises a stl::list of Particles using a Fortran callback function
that uses the picker module to establish the initial cell and local
coordinates of a detector.
Detectors: Implementation of a simple .stat file detector writer

This requires the callback functions to evaluate scalar and vector
fields at the particle position and currently only works for
non-binary files.
Detectors: Adding binary output and remaining initialisation modes

Lagrangian detectors are not yet implemented, so static ones are
created instead.
Detectors: Add LagrangianParticle class with RK4 advection algorithm

Basic implementation of the original lagrangian advection algorithm,
including stage computation and parametric coordinate tracking.
Currently only performs RK4 from static parameters and probably
wants some more optimisation.
Detectors: Detect if a particle is leaving the local sub-domain

Now we only need to send it to its new owner and continue tracking.