Immersed boundary

This fork completes the implementation of the immersed boundary
method for simulation of elastic membranes in the Espresso Simulation package.

The code was created by combining the lattice boltzmann method as
implemented in espresso and a discretized capsule mesh which interacts with the fluid via
the immersed boundary method.

In this pull request there is the virtual particles that are interpolated with the fluid (lbtracers) the trielastic membrane force (triel) and finally a volume conservation of cell membranes in (vvolume)